Most mons - generally speaking - don't want to run Rocks, particularly among OU's strongest. They run rocks as a "necessary evil" but I find this gen particularly, It leads to some key MUs being sacrificed - esp with Tera being accounted for.
- Clefable IMO is not great Rocks user. I find that it rarely finds oppurtunities to setup rocks in the first places (gets pressured too easily + can struggle with some of the removers like Treads, Glimm, Cind, Hatt while not being amazing into Tusk either). That said, I also find these utility sets in general to be much weaker than CM sets - which are more easily able to win the game and has better match-ups into threats such as Raging Bolt, Walking Wake, Kyurem, etc.
- Garganacl is a decent Rocks user, having good MUs into most of the common hazard removers. That said, I find these Rocks sets Garg typically runs to be exponentially easier to handle than Curse Garganacl, which can set up alongside some of the key threats Garganacl is tasked with handling, and defeat much of its typical counterplay.
- Great Tusk and Iron Treads are component Rocks setters, but you typically need to sacrifice running a key move on these rocks sets, usually Knock Off, which really hurts both Treads / Tusks MUs into key switch-ins like Corv, Moltres, Zapdos, etc. Both mons have a lot of other useful moves they would rather run as well - CC for Tera Normal Gliscor, Supercell Slam for Tera Water mons / Corv + bolt beam coverage, Temper Flare occassionally, etc. I personally hate it when I am forced to run Rocks on Great Tusk.
- Tyranitar is a decent Rocks setter - i guess, but I find slotting that mon in general to be a bit tough + I feel it already has 4MSS as is (you want Low Kick for Gambit, Rock move for Kyurem / Moth / birds, and Knock off already, and an Ice move or thunder wave occassionally. Even then, I don't feel its easy for this mon to set up rocks at all given its general poor MU spread into other big threats like Lando-T, Valiant, Samurott, Ogerpon-W, Zamazenta, etc so I typically don't get many oppurtunities to click the move when I run Ttar.
- Gliscor / Ting-lu are fine Rocks users, but they usually need to drop a move (in Ting-lu's case usually Earthquake for double hazards, while Gliscor needs to drop Spikes - which isn't ideal).
- Glimmora is decent but typically relegated to HO.
- Tinkaton is decent rocks setters, but can be annoyed by Treads, Tusk, Cind. Still, it usually has all the utility it needs on rocks sets.
- Landorus-T is good rocks setter, probably one of the most ideal ones. Could be argued that dropping Stone Edge or Grass Knot (or Psychic for Zama / Tusk) isn't ideal, but not much can be done there.
- Heatran probably is one of the best Rocks setters because isn't really dropping any important move it needs - MAYBE Will-O-Wisp, but Heatran mostly is fine with Fire Move / Earth Power / Taunt, so being able to setup Rocks for it is pretty huge.
- I normally don't use Deo-S. It is pretty decent entry hazard setter from what I've seen, but typically on more HO focused teams, usually running it alongside Dual Screens.
My point is that most mons in SV OU feel like they are sacrificing a lot of key MUs by running rocks. For me personally, I feel that delving into the more niche rockers is worth it so mons like Gliscor, Great Tusk, Clefable, and Garg can run their most crazy busted sets compared to relying on them as Rockers. Now, you could argue this is the case with Solgaleo as well, but I feel solgaleo could have run a variety of rocks sets using its wide movepool to give bulky offensive / balance teams an additional tool to work with - with Steel STAB to pressure Hatterene, Knock Off for Cinderace, and Psychic or a Fire move for Great Tusk / Treads, I feel such a set could have provided a decent combo of defensive utility for most standard teams while maintaining the rocks up and not losing much for it. A set as mundane as Stealth Rock / Psychic / Flash Cannon / Fire Blast with a type resisting berry basically covers everything you want it (Great Tusk, Glimmora, Hatterene, Iron Treads) in addition to checking a decent swath of the big threats like Kyurem, Enamorus, Iron Valiant, etc, all while not being useless in other MUs such as against Ghold / Gambit. Alterantively, CC or Focus Blast could be run for Samu / Ting-Lu. Or on balance teams, you could have run a Rocks Morning Sun set with SD Gliscor as wincon, which can help against Kyurem / Gweezing that can be annoying for Gliscor. This means you only need to delegate one other slot to spikes, whereas current configures of Solgaleo + Gliscor need to dedicate two slots to both Spikes and Rocks or run both on another Pokemon like Ting-lu (which has good synergy with Gliscor, but does leave you vulnerable to similar threats).
As for Rachi / Empoleon, I can't speak towards Jirachi (but from what I've seen it is solid), but Empoleon has been a pretty goated alternative rocker on a bulky offensive team I like running. Blocks Parting Shot from Pecharunt and Mortal Spin from Glimmora in addition to luring and KOing Great Tusk / Lando-T with help from Shuca Berry, even punishes Defog hard from Corv. It brings some unique resistances too, namely its resistances to Water and Dragon, which imo is pretty notable given that Primarina can be a trade machine against some bulky offenses. Its definetly not perfect - it too has some of the worst 4MSS I have ever seen in a mon since you want Rocks / Knock / Flash Cannon / Ice Beam / Air Slash / Surf / Roost / Roar / Haze, etc., its stats aren't the best either, but it has impressed me far more than I was expecting.
That said, I do think I did overstate how Solgaleo would be "difficult" to fit compared to Empoleon / Rachi. Rather, I think the correct phrasing is "more boring". A lot of the sets that are popular just don't seem very fun or interesting to use even if they are effective- generic bander (with bad STABs so using it is annoying), infernape Larp with random 4 moves as coverage + a shiton more bulk (which is admittingly nice), generic Veil cheeser that gets phased out and screwed over by Ting-Lu like its buddies Moltres-Galar, Raging Bolt, Iron Moth, Latias, etc. Trick Room + Teleport is admittingly cool, though it does suck that Solgaleo itself doesn't benefit much from the TR (plus I am not sure how much better this is that something like Lagging Tail Slowking, but I'm not a TR player). I genuinely thought what would have made this mon stand out as a nice option in the builder was those alterative utility tools - with stuff like Colbur or Shuca to help check certain threats like Darkrai, Gambit, etc. or running Roar + its collossal bulk to phaze out key threats on veil / webs (which is pretty big), but these don't seem to be common options, which is a shame. So far, in both these offensive and defensive roles, what's mainly let Solgaleo stand out is its ability to trade + difficulty to scout - which are both no doubt valuable in its own right, and I think I agree with others assessment this mon would probably be in the A/A- range based on these alone. But its also not as tier warping as I was hoping (specifically in the ability to check threats + condense utility where I feel its more comparable to Empoleon / Jirachi, than something like Magearna).