Played through Ultra Sun recently and finished the game, so figured I'd post my experiences here:
Rowlet: B Tier
After hearing so many people talk about how Rowlet was awful for them in-game and that it kept on randomly dying to coverage moves, I was resigning the idea of using it to be a horrible experience. However, it actually really pulled its weight, and surprised me at how well it performed as both Rowlet & Dartrix. Supersonic Skystrike coming as early as it does is a godsend for Rowlet, and gives it a Z Move nuke option at a really early time, letting it cheese its way through matches it otherwise might not have any chance winning. I really feel it goes quite some way to compensating for Rowlet's lack of powerful STAB until very late game, and it made some totem battles, particularly Lurantis (and Araquanid potentially, though I didn't use it there), a total joke.
It definitely felt a bit worse to use upon evolving into Decidueye, but it was bulkier than expected, and never got 1 shotted by random Dark/Ghost coverage as many claim, and it gets Sinister Arrow Raid right around the time where the loss of SSS becomes really noticeable. Honestly the biggest knock I can put against it is that since mine ended up being Jolly, I couldn't get the Giga Drain tutor, so I was stuck with Razor Leaf until Ula Ula, well into its time as Decidueye, which made it feel a bit weak at times. It became more of a utility player around that time because its typing made it really vulnerable to Team Skull mons, but it was never totally useless, and pulled its weight even in the Elite 4. The move tutor options and SSS after the 1st totem make it exponentially better than it was in S/M imo.
Grubbin: A Tier
Another case of a Pokemon being flat out better than it was in S/M. The Grubbin line was frankly, painful to live through, with incredibly low stats letting down what is otherwise a surprisingly nice movepool for a Route 1 bug, as it only really had Mud Slap to contribute to big battles. Once it evolved into Charjabug however, the tables completely turned, and it immediately became the most consistently good member of my team, and never relented. Had great stats for the point of the game it was in, proving strong and surprisingly tanky, with moves to take on pretty much any situation. And right about the time it'd normally fall off as my mons become fully evolved, Blush Mountain arrives and it evolves into Vikavolt, which is such a vast improvement from S/M forcing you to wait until Vast Poni Canyon, and while still a bit slow, it never stopped destroying everything it was up against with base 145 Special Attack Thunderbolts, Signal Beam for any Electric resists, Electrium Z for any nuking situations, and shockingly, it managed to outspeed quite a lot of foes by virtue of random Speed EVs it picked up early game.
The only real "problem" it had was that move relearners can't be used until you're literally on the Elite 4's doorstep, which meant it couldn't learn Bug Buzz and got stuck with Signal Beam until right before the Pokemon League. But even with just Signal Beam and Thunderbolt, it more than pulled its weight throughout the game, never had a slog period upon becoming Charjabug, and just kicked ass.
(as an aside, you can get a Charjabug at Blush Mountain and Rare Candy it to immediately get a Vikavolt on your team as well. Thoughts?)
Meowth: D Tier
Remember the problem I mentioned with Rowlet where it was stuck with Razor Leaf until Ula Ula Island? This cat has that problem nailed down a hundred-fold. It started out incredibly well, being fast and coming immediately with Technician-boosted Bite, making it an early game MVP which easily 1-2 shotted anything it came up against, with Shock Wave and Work Up giving it a little more firepower if Bite didn't work. Little did I realize that Technician Bite would be the pinnicle of its STAB until fucking POSTGAME - it evolved into Persian on Akala, and performed nicely up until after Totem Marowak, where its sheer lack of good moves and abysmal Defenses crippled it for the rest of my run. If you want an idea of how poor its STAB is, look at its level up movepool, then realize Dark Pulse is post-game. I'm not exaggerating when I say it never improves from Technician Bite.
This thing died constantly, almost never OHKOed its foes starting from Ula Ula, and worst of all, got barely any better options as the game marched on. Power Gem worked nicely against mons 4X weak to Rock like Guzma's Masquerain, but if it didn't KO, it got 1-shotted swiftly and without hesitation. It got Nasty Plot, but no STAB to use it with, and was so weak it'd only sometimes KO even with the boost. It had so few options that my set going into Mt. Lanakila was Nasty Plot, Shadow Ball, Shock Wave and Power Gem - not even a STAB move! (Though Dark Pulse is at least right before the E4) And it must've had awful IVs to top my horiible experience using this, not just because it couldn't ever take hits and struggled to OHKO enemies, but against Hau, it got outsped by his Tauros, despite being 4 LEVELS HIGHER. It at least got to sweep Acerola and a number of lower leveled trainers, but it was otherwise the weakest member of my team before I even had a 5th member, and only occasionally got to strut its stuff. Thoroughly disappointed and frustrated with this cat.
Passimian: B Tier
Not much to say about this guy - it's SRable, unlike in S/M, so finding one isn't anywhere near as much of a problem, and you can immediately give it Brick Break, Rock Tomb, Brutal Swing and Fightnium Z for a nice makeshift moveset right off the bat. It was incredibly strong, a reliable option in almost any battle, and almost always made its mark when used. At times late game, it couldn't take special hits especially well, and it was stuck with the moveset listed above and Bulk Up for almost the entire playthrough (Close Combat and Rock Slide emerging later on), but the moveset itself offered great coverage, and the sheer strength of Fightnium Z coupled with its relatively positive matchup spread across the game can't be ignored. Solid, but not spectacular choice.
Mareanie: C-D Tier
On paper, this Pokemon should be utterly abysmal for an in-game run - SOS only, slow, poor offenses, stalling focused, fairly late evolution, no Water STAB until level 58 (or the Scald TM), and to top it all off, mine ended up being Adamant. And while it did need a little bit of grinding to start off with as a Mareanie (only about 2-3 levels), this Pokemon was full of surprises, and did a lot of good for me in my run, even if the 3/4 of its run it had no Water STAB whatsoever.
As a Mareanie, even with an Adamant nature and bad offenses, Toxic + Venoshock did a surprising amount of damage to most foes while also offering plenty of residual damage on the side, and it later got Recover to aid in stalling, as well as Poison Jab to both do damage and poison foes simultaneously, as well as Payback to make use of its rock bottom speed. It did die quite a lot as Mareanie, but still outperformed the rock bottom expectations I had for it.
It wasn't until it evolved into Toxapex where this thing actually became an MVP in its own right - its titanic defenses and great defensive typing let it resist nearly any move enemies could throw at it, making it perfect to throw out and heal the rest of my team, and upon evolution it gained enough offensive presence where Poison Jab (later Acid Downpour) could actually OHKO outright on SE hits, and Toxic Spikes helped make harder battles much easier for the rest of my team to deal with.
The main reason I speak so highly of Toxapex however, was that thanks to Poison (and Pokemon Refresh, admittedly), it singlehandedly dealt with Ultra Necrozma on my 1st try, who would've otherwise annihilated me to kingdom come multiple times over. And it's not as if it can't do work in regular battles, as it could outlast and beat most anything it came up against, so long as it wasn't a ground, psychic or electric type. I absolutely think this thing is significantly better than something like Larvesta (which is 1%, total shit and likely won't evolve until postgame), Yungoos (abysmal at any stage of the game), or even the mon I'll be talking about below, and I'd go as far as recommending it to go up to C Tier, given where the tiers stand right now. It's not like Mareanie is all about poison stalling - it can do that, but it can also provide offensive support in its own right too.
Turtonator: D Tier
Talk about a disappointment - I really wanted to use this guy, but it sadly ended up being by far the most expendable member of my team. It also being Adamant can't have helped my opinion of it, but it was just as slow as its stats would indicate, without being anywhere near as bulky as its huge defense would imply, or as strong as it looks. It coming with Flamethrower was sick, but it had to use Charge Beam and Stone Edge for early coverage, and it just wasn't strong enough to effectively wield any of its attacks. It couldn't even utilize Shell Smash effectively because of how slow it was, and the defense drop usually ended up with it dying if its opponent survived. It was nice as a pivot at times, and was great at handling anything it could resist, but most of the time all it did was spam Flamethrower or Overheat with Firium Z in fights it could win, and it almost always lost against other Dragons due to its pitiful speed. Passimian beating a few of the same Pokemon Turtonator handled while also outspeeding them also probably didn't help Turtonators case, but I honestly wondered why the hell I picked it up by the time I reached Mt. Lanakila and rarely used it for much of anything.