WHY DOES SOLGALEO SUCK ? (AND THUS SHOULD BE SUSPECTED)
Despite being one of the box legendaries for Sun and Moon, I have the utter conviction that Solgaleo would be completely fine in Gen 9 OU, and not even just that, I believe it wouldn't even be that good in OU (UUBL AT BEST !!!!!). You may think I'm crazy, and you may be right about that, but not for the reasons you might imagine. Let's take a closer look.
At first glance, this mon has great stats ! Amazing bulk and pretty great attack, with meh speed, but still usable, after all you're faster than landorus-therian and kyurem, which are both OU mainstays ! Sounds good for our glorified metagross, right ? But those stats
mean nothing if you don't have the tools to use them.
1. Typing
Let's start with
Solgaleo's typing. Steel + Psychic is a
notoriously terrible stab combination, leaving you
unable to neutrally hit Steel types (Dengo, Kingambit, Corv, Iron Treads for example who are all meta defining mons)
or H-Samurott without slotting a coverage move. This effectively means that your stabs are never really free to click, leaving you in an awkward spot of always trying to guess the switch or be forced out, either by a bulky support threatening status (
Alomomola for example
completely walls Solgaleo) or an offensive pokemon coming on a resisted hit to force you out (Dengo or Gambit). And while Solgaleo does possess great coverage options in Close Combat, Earthquake, Flare Blitz, Knock Off or Wild Charge, it suffers from
heavy 4MSS. But we'll get back to that later.
Solgaleo's typing is also downright TERRIBLE defensively, which severely undermines his natural bulk. Being weak to
ground, fire, ghost and dark which are some of the
most common offensive types.
Iron Crown shares the same double-typing, but it has a much better speed tier (if you use booster) + an actual pivot move that lets it get momentum when it's counters come it (that allows it to run a good AV set, unlike solg). Solgaleo can do no such thing, as pivoting via Teleport leaves you open to getting taking big damage on the way out.
2. The speed tier
97 is not an horrible speed tier per se, but it's not great at all in today's standards (especially for an offensive pokemon) just missing the golden mark of base 100s. But don't let this vague and outdated hallmark convince you: Let's take a look about every offensive/similar OU mon that's slower/as fast as SolG and draw a comparison ! Perhaps then you'll understand why Solgaleo is unviable trash while they flourish despite their speed tier.

Has the most broken move in the game in Ceaseless Edge, two priority options (Solgaleo has none), Swords Dance (Solgaleo can't setup raise his attack), Encore and Taunt (Solgaleo), effectively higher attack when using it's stabs due to Sharpness, better typing offensively and defensively, gets flip turn to pivot (solg has tp but pivot attacks are better for offensive mons)

Incredible ability that enables disgusting hstack structures, better typing offensively (and kinda similar defensively, but still better due to fighting immunity), better stab options with similar offensive stat, thunder wave, can run specs scarf defensive offensive balloon etc etc

Broken ability in multiscale giving it a free turn vs almost anything, better coverage options with
similar offensive stat (ground ice normal or dragon fire normal hits everything), can raise its speed anyways with dragon dance (still no set up moves for solg), amazing prio in espeed, can run bulky sets with roost (solg can do the same with morning sun but we'll get to why that isnt good later)

Broken ability in poison heal which makes it effectively way more durable over the course of the game despite having lower bulk, amazing defensive double typing giving you a double immunity to help patchup your other mon's weaknesses (whereas solgaleo gives you more weaknesses to patch up), swords dance and agility, gets stealth rock and spikes while solg can't set up hazards, toxic while solg has 0 moves to inflict status, gets uturn (can use both fast or slow depending on set, while solg can only slow pivot and doesnt have ability that lets him heal for free after taking dmg to pivot)

Sets up grassy terrain for himself which makes it effectively WAY stronger offensively than Solg when using grass moves, absurdly strong priority, can set up with swords dance too, gets uturn

Similar attacking stat but gets to snort booster energy to push it way past, great priority option in thunderclap, almost perfect neutral coverage with elec + dragon so no need for coverage, can set up with calm mind (solg also gets calm mind but it has no prio and much lower special attack which means its not really comparable, although solg could set up and heal which bolt cant do), can pivot with (an absurdly strong) volt switch

Similar offenses to Solgaleo's attack in both physical and special, making it able to run terryfing mixed sets with a MUCH MUCH MUCH better offensive type and perfect Ice/ground coverage (only walled by cryogonal and levitate bronzong kekw), can set up with dd, can be a fuckign bitch with subtect pressure lefties, AND has lower but similar bulk

Bro do I even need to say anything?????
I think you'll find it clear now that all these mons slower or with similar speed to Solgaleo all have some incredible traits that stand out and make them worth running that go beyond simply having "great overall stats" on paper, like Solgaleo.
And that's only including mons with roles that could be at least similar to Solgaleo, as I'd find it pointless to compare it with Slowking-G for example.... UNLESS ??????
3. The futureport delusion
A common argument I've been hearing against a Solgaleo retest is that it could run a futureport set, similar to slowking-G. However you might have noticed there's a smmaaaaaaalll difference between these two mons...
The whole reason glowking works well is because you can fsight, then chilly out, and regen all or part of the damage you took on the way out because of regen.
This lets you generate momentum and offensive pressure while conserving health naturally.
Solgaleo cannot achieve this, as everytime you come in to futureport, you take chip which you won't naturally regen. This means you need to slot in Morning Sun, which is already very annoying as that only leaves you with 1 free slot (and solgaleo is strapped for moves already). This also means you're much less effective at getting momentum, because
you need to also spend turns CLICKING Morning Sun to stay healthy, turns during which something may switch in and force you out, preventing you from getting a Fsight up at all.
Let's move on.
4. The movepool / 4MSS problem (why Solgaleo can't use any of its good moves effectively)
Solgaleo has a pretty good movepool in theory. Its main stab is
Sunsteel Strike, a steel-type earthquake that ignores all abilities à-la mold breaker. In theory this would make it good at pressuring unaware mons... except Solgaleo can't increase his attack, so that's a moot point. Its other stab (psychic fangs) is mid, but Solgaleo gets great coverage moves:
Close Combat, Flare Blitz and Earthquake are all very strong both in BP and in the typing coverage they offer! It also gets
Knock Off to force progress ! You even have
morning sun à-la gouging fire with a similar stat profile ! What's not to like?
Well, in practice, dropping one of your coverages to fit in morning sun is very tough on a physical set. For example, Sunsteel Strike/Psyfangs/CC is HARD walled by gholdengo and corv (252 Atk Solgaleo Close Combat vs. 248 HP / 252+ Def Corviknight: 95-112 (23.8 - 28%) --
possible 5HKO), + not running Wild Charge also means you can't hurt Alomomola (252 Atk Solgaleo Psychic Fangs vs. 252 HP / 252+ Def Alomomola: 120-142 (22.4 - 26.5%) -- 25.1% chance to 4HKO, and that's not even mentioning Dondozo).
Dropping CC means you're now walled by H-Samurott and you also can't ohko Kingambit which threatens back hard. If you drop psyfangs, you are now walled by Pecharunt and Pex... and it gets even worse if you try to run both morning sun and knock off. So you can pretty much write off 3 attacks + msun as a cheese pick vs offense which might not even work that well as a lot of mons can actually deal a shitton of dmg to this despite its natural bulk, for example:
252 Atk Protosynthesis Roaring Moon Knock Off (97.5 BP) vs. 0 HP / 0 Def Solgaleo: 408-482 (98.3 - 116.1%) -- 87.5% chance to OHKO
+1 252+ Atk Dragonite Earthquake vs. 0 HP / 0 Def Solgaleo: 346-408 (83.3 - 98.3%) -- guaranteed 2HKO
32 Atk Cinderace Pyro Ball vs. 0 HP / 0 Def Solgaleo: 288-338 (69.3 - 81.4%) -- guaranteed 2HKO (+ ace also threatens with willo)
252 Atk Great Tusk Headlong Rush vs. 0 HP / 0 Def Solgaleo: 372-440 (89.6 - 106%) -- 37.5% chance to OHKO
This comes back around to Solgaleo's defensive typing and midling speed being a huge hurdle when trying to use it offensively.
The other option is to run a CM set, trying to leverage your huge bulk, but you'll run into the same issues. Morning Sun + CM is required on a set like this, and psynoise is probably the main draw. So you have to choose between flamethrower last and lose to samuh, or focus blast last and lose to gholdengo.
5. Item options (and why they all suck)
- Band hits hard in theory, but because your stabs are extremely bad typing wise, in practice you can never click them so you can't actually break effectively. Getting a wrong predict and clicking Sunsteel Strike into corv/mola is much worse when running band than lefties or something else. And even nailing the correct move doesn't always reward you :
252 Atk Choice Band Solgaleo Wild Charge vs. 252 HP / 252+ Def Alomomola: 254-300 (47.5 - 56.1%) -- 84% chance to 2HKO
- Scarf can fix your offense matchup, but then you become even more hardwalled by all the common ou goons (mola corv dengo etc), and you also become a huge liability vs setup mons (dnite or gambit)
- Expert Belt or LO lets you make good use of your coverage, but then you become very prone to chip from hazards/your own LO and as you're not that fast, in practice you'll get maybe one big chunk of damage (if that) vs a defensive pivot then get forced out by a faster attacker and have to take hazards again when coming back in.
- Boots fixes the problem from above but again goes back to being unthreatening vs any good defensive core.
As you can see, none of these options seem really satisfying.
6. Conclusion
Solgaleo, while scary in appearance with its big stats and shiny signature move, is in reality nothing more than a cowardly cat and he could roam free in the land of OU without making much impact in my opinions. This post might not be perfect; after all, Solgaleo has so many flaws that's there's probably a dozen more I could have pointed out !
TL;DR: Suspect Iron Mufasa so we can all laugh at how bad it is.