Ok I think there are some points I'd like to address about Victini that may make it not worthy of S-Rank like you've said.
Ok, first, we have V-Create's drawbacks. Yes, I know you did mention them, but I think you need to address the negative effects of V-Create and their outcomes in a little more detail. Firstly, it really fucking hurt Scarf's ability to clean late-game. One Speed drop basically causes Victini to lose its Scarf while still being locked into its STAB, while another one makes it really slow and easily revenge killed. I know Victini can use Flare Blitz as an option, but if A. You have to sack a coverage move in order to do this and B. Flare Blitz is outclassed af by Darmanitan for obvious reasons (Sheer Force, higher Attack stat, etc.) As such, I really find that Scarf Darmanitan outclasses Scarf Victini a good amount.
Second, I really feel as though you're over-estimating Victini's flexibility when it comes to all of its sets. A lot of the sets you listed are... well, average at best. Honestly, it's main sets are Choice Band, Choice Scarf (Who I've already stated I feel is outclassed), Physical Life Orb with Energy Ball, and Special Life Orb. Shit like TR Tini and Work Up Tini really can't be used to say that it's so much more flexible then Darm... I mean I know it definitely is more flexible then Darm, it's not as unpredictable as you make it out to be.
I also feel like you're really underplaying Stealth Rock. Yes, we all definitely understand that Defog/Rapid Spin users are much more common this Gen. However, you failed to mention how A. Spinning/Defoging loses serious momentum for your team, B. A good chunk of the common Spinners struggle to beat a lot of defensive Ghost-types, C. The opponent can always set SR up again to hurt Victini and force you to spin again, and D. A lot of the Spinners/Defog users are prone to being worn down quite easily.
Finally while Victini can definitely beat Bulky Water-types much easier, Darmanitan has a much easier time against defensive Arcanine, something that Victini struggles against.
So anyways while I do believe Victini is overall better than Darmanitan as a Life Orb/Band user, I really don't feel as though it's enough to warrant S-Rank. I'm not the most active UU player, though, so if some of my points are wrong/not very accurate, feel free to address them. Though I think you did make some compelling arguments that need to be considered.
While I certainly appreciate some of these points, I can't say that I agree with them entirely. As I mentioned, a speed drop doesn't cause Victini to "lose" its Scarf. Victini's Scarf is used in order to come in on fast offensive threats and scare them out with the threat of a V-Create, forcing them to sack something or switch into something defensive - and in either case, the speed drop is mostly irrelevant.
Second, I honestly don't find any of the sets I posted to be average whatsoever. Nearly all of them have been used in recent tournaments (like the UUPL) - just because something doesn't do the massive damage of the band set doesn't mean it's not viable or not good; often, surprise factor is immensely important in distinguishing A ranked pokes from S ranked pokes, and if a poke can viably run moves that give it surprise factor, it's a big boost in its viability. Even if we ignore all of the "gimmicky" (but actually quite good) sets and say that Victini can only run combinations of special and physical movepool (e.g., choose any four - V-Create, Blue Flare, Bolt Strike, U-turn, Grass Knot, Psychic, Psyshock, Zen Headbutt, Final Gambit, Focus Blast), it still isn't "predictable" enough to be countered by anything at all, except for perhaps fully defensive AV Snorlax with pursuit which A: isn't exactly common and B: doesn't exactly enjoy taking two banded V-Creates, if that ends up being the case.
252 SpA Life Orb Victini Psychic vs. 252 HP / 4 SpD Arcanine: 191-226 (49.7 - 58.8%) -- 69.5% chance to 2HKO after Leftovers recovery
-1 252 Atk Darmanitan Earthquake vs. 252 HP / 252 Def Arcanine: 140-166 (36.4 - 43.2%) -- 98.2% chance to 3HKO after Leftovers recovery
Finally, in order to address your Stealth Rock/hazard point, I'm going to bring up a little comparison that I think is relevant - Victini's place in the current metagame as opposed to Victini's place at the very end of Generation 5. For those of you who were around back then, you'd know that Victini actually went through an extremely controversial suspect test last gen, and it was absolutely considered one of the premier pokes of UU. So, what changed from now until then?
1. Defog/Spinners in UU. Like I said in my last post, removing hazards now is a cakewalk
compared to what it was in gen 5. Whether or not it's easy to get defog or spin off in the objective sense of the word (which I'd honestly still argue it is, but that's mostly irrelevant), it's absolutely far easier than it was before. In that regard, this generation shift absolutely benefited Victini.
2. The dreaded Knock Off. Okay, this one clearly wasn't friendly to tini. The dark buff in general means that we see slightly more dark moves in UU than previously. However, if we look closer, we realize that as much as people gripe about Knock Off being broken, it really isn't affecting Victini all too much. Let's look at pokes that commonly carry Knock Off in the metagame.
Mega-Sol: He certainly can't come in on Victini. If he switches into Victini after sacking something, it's more of a 50/50 guessing game as to Sucker Punch or Pursuit/SD than it is about Knock Off. However, his presence is slightly annoying.
Krookodile: Fully defensive Krookodile is annoying to Victini, as even U-turn doesn't do all too much. At the same time, it doesn't appreciate taking an LO grass knot.
252 SpA Life Orb Victini Grass Knot (80 BP) vs. 4 HP / 0 SpD Krookodile: 255-302 (76.8 - 90.9%) -- guaranteed 2HKO after Leftovers recovery
Offensive versions will be slaughtered by a V-Create, and Victini outspeeds, so Knock Off isn't really all too relevant here either.
Machamp: Can't switch in at all, and Band straight OHKOs.
Mew: Only fully defensive Mew can risk taking a banded knock off, and Knock off only does 37-44% anyway.
Mienshao: Knock Off does offer a slightly more reliable way than Stone Edge to kill Victini, but prior damage is needed even on Jolly LO to ensure the kill. Stone Edge still does more damage, too.
I might be forgetting a couple, but the point still remains the same. As good of a buff as dark types got this gen, Knock Off specifically didn't really effect Victini all too much. At the end of the day, all it means is that Victini has a bit harder time switching into Mew and has to be slightly more careful around Mienshao.
3. New drops from previous OU: This would be relevant, if there was really anything that got dropped that can aptly deal with Victini. Hippowdon is decent, but he doesn't enjoy taking grass knots to the face.
In other words, the generation shift didn't really seem to hurt Victini whatsoever, if at all. If Victini was considered an S-rank mon (easily) last gen, what has really changed this gen? As far as I can tell, from a purely anecdotal perspective, Victini's potential is being utilized even more and more effectively this gen than it was last gen.
EDIT: Oh right, Heracross. Tbh, Knock Off is just slightly more safe to lock into than EQ/Stone Edge were last gen, and there's less of pursuit heracross running around as a consequence, so this one might even be a net positive.