Funny, I was about to start a very similar thread to this!
I haven't been into AC since 4, and I had wanted to get back into it, so I started with AC3 on the Switch.
3 was always my least favorite of the ones I've played (1-4), but I wanted to give it another shot in case I was misremembering or my hype when it first came out clouded my judgement.
Unfortunately, it was about as bad as I remembered it.
The game takes like 5-6 hours before you start playing as Connor as a grown Assassin and the game fully opens up. 3-4 hours are playing as his dad, and then the rest is as young Connor.
The game also never lets up with introducing new mechanics or systems that you never have to touch again. Hunting, the caravan bartering system, buying anything at all (there wasn't a single thing I found I wanted or needed to buy other than maybe ship upgrades), the ship missions (there's like 2 you need to do and the other handful are just side content which to be fair are the best missions/levels in the game), the "summon assassin's" button feels really tacked on in this game and only really need to use it once or twice, etc.
It's like half the mechanical stuff in the game doesn't really fit this game, but since it was in past games then they had to find ways to cram it in. And the ship combat stuff is some of the best parts of the game, but it doesn't really fit with your character to make him a ship captain out of nowhere. It really feels like they rushed that in since they were already working on it for AC4 and they knew it'd be fun.
The cities really disappointed me too. Only Boston and New York as full cities, and a large wilderness zone that just becomes a pain to navigate through so I end up fast traveling past it. The cities aren't really fun Assassin's Creed cities since they have such wide roads which makes rooftop traversal much harder, if not possible at all for large stretches. The landmarks are also lacking compared to past games. Sorry colonial America, but random churches and a handful of notable wooden buildings aren't that impressive compared to what Italy has.
In my replay I was most looking forward to playing as Connor to see how his story pans out. I barely remembered any of it since I cared more about the Desmond storyline's conclusion, but in replaying it I found it as forgettable as the first time. He's got one good conversation with Samuel Adams (I think) where he argues about slavery and freeing ALL people, not just the whites from British rule, but that's about the only noteworthy one.
His whole motivation is killing Charles Lee who killed his mom, but he runs into him multiple times in the game and doesn't kill him for really stupid reasons, until the end of the game where you just finally get to do it because the game says so.
The Templar/Assassin storyline wasn't great either. The Templars don't really have much of an evil plan this game, it's just Charles Lee that went rogue and killed the Native American village. Their main goal is just stopping the revolution, then wanting to lead the revolution when they fail at that. And Connor doesn't really care much about anything that's happening, he feels like an errand boy for all these historical figures in a way I didn't feel from any other AC protagonist.
And good god that conclusion is still the WORST. An incredibly easy to fail chase mission where you eventually catch the guy, shoot him, then he takes a ship to a random bar, where you just happen to find him, and THEN you kill him.
The Desmond conclusion isn't great either, but they had to wrap it up somehow I guess. I still kinda miss Desmond as the player surrogate in the modern day times, but I guess they just ran out of stuff for him to do.
I also decided to do the Homestead missions this time which I completely skipped the first time I played it. They're fun missions and I enjoyed recruiting people and doing the little missions for everyone, but at the end it felt like none of it mattered. I was invested in a romance story between two of the characters, and I did every missions I could find between every main mission, but in the end there was no conclusion, the guy gave the lady a knife and that was it. And overall I never found any benefit or purpose to doing any of the missions, it just felt like yet another tacked on, half baked system crammed into the game.
I still had some fun playing it, but about 10 hours before the end I was definitely getting bored. The side content is definitely the best stuff in the game, like the peg leg ship missions and the homestead missions (even if the latter doesn't really go anywhere). The game just felt rushed IMO. The tree climbing was a nice addition but feels a bit clunky, and the game being so filled with systems that it does nothing to encourage you to use more than once ends up making it feel bloated.