Alright, I just watched this movie. It is stupid and several effects are bad. It's not really a good movie by any means but it's barely worse than the first one, so if you liked that one it's not like this one is going to piss you off. A good chunk of this movie could easily be considered filler and quite a bit of that is about the sister-in-law getting married, which just felt tacked on in many ways.
As for any details, in case anyone is still wondering, much of Knuckles' characterisation in the trailers is misleading and a few lines are missing. Knuckles in the actual movie feels like some bizarre mixture of what he was like in Sonic Boom (oblivious and slow witted) and Piccolo from Dragon Ball (brooding loner warrior type).
Some of the dumber bits in the movie is the sister-in-law speculating that the person who waxed her (what is implied to be) pubic hair was spying on her, and Knuckles suggesting they should attack Eggman's groin. Speaking of Eggman, Sonic calls him Robotnik more than he does Eggman, which is weird to me.
It's not made entirely clear, but it is possible that Sonic, Knuckles and Tails may in fact be from different planets rather than the same one. I'm not sure if that's the implication, the Echnidas, Owls and Sonic all seemed to be from the same planet in the first one. This one however makes it clear that the owls traveled around the universe, Tails specifies "his" planet rather than "their" planet and several characters and factions are referenced in relation to the galaxy or universe.
As for spoilers:
Spoiler:
Eggman escapes the mushroom planet by contacting human sized humanoids in masks, seemingly from the same planet as Knuckles. I don't know what to make of that, I know Sonic X had humans on Sonic's world apart from the ones on Earth. I guess that these movies draw from Sonic X more than any other adaptation but in kind of a loose way. To be honest Sonic X itself is kind of confusing in the regards to the multiple planets and the creatures living on them, I'm not sure what to make of either. Maybe they were these seperate humans, maybe they were they were just aliens, who knows? I think the universe is meant to be a mix of alien humans and furries like in Sonic X but it's hard to tell.
The Chaos Emeralds were used to forge the Master Emerald and, up until the climax, were contained within it. When Sonic scatters the emeralds, Knuckles takes the shards and repairs it, so now it exists as it's own thing apart from the Chaos Emeralds and now it's being kept in a a cooler. Not sure if that means it's less powerful or not. I'm guessing this is the explanation for why Knuckles can travel around and still guard it, something the games never quite explained (though I guess SA2 showed that he could just shove it up his ass). There is no mention of Chaos or a puddle sliding around when the Master Emerald breaks as far as I could tell.
Yes, Shadow is in the after credits scene. Much like in Sonic X, I don't know what to make of it. I can't tell if he's an alien like Sonic or if it's just a coincidence. It's implied he has some connection to Eggman, meaning the Ark is probably a thing in the third movie.