Dawg I've done 180 emblems quite recently, I genuinely don't know what you're talking about in regards to "fluid gameplay".
Getting sucked into a wall because I dared to veer slightly to the right? The homing attack not working half the time? Light dash rings being a crap shoot because your angle was off by a pixel, and god forbid you get a lightning shield before those sections? Bounce attacking into a god damn pit because the light dash activation zone around a ring is almost the same size as the rings physical hit box? Rail hopping just not working in general? Aiming with the mech being all over the place and its wishy washy controls that try to emulate a tank but just make it inconsistant as all hell in its movement? How the game constantly feels like its going from a snails pace to mach 7 because the controls always seem to jump from "sluggish and unresponsive" to "touchier than Jared Fogel", the mech characters being the most poignant examples of this? Almost every A rank requiring a very specific path because point combos are largely more important than speed IN A SONIC GAME?
Rouge and Knuckles are so close to the "fluid gameplay" you seem to refer to, but then they stick to walls like a fly on shit at the worst of times, get awkwardly stuck on geometry while climbing all the damn time and the camera fights them way harder than any other characters. Instead of having a speed objective or obstacle course with a finish line to reach, more like the other two sets of characters to make use of their unique movement options, you got them doing easter egg hunts in tedious enclosed levels that somehow feel too obtusely large and cramped at the same time. Pumpkin Hill, Meteor Herd and Mad Space being the exceptions to cramped, and Mad Space is just straight miserable even when you know the layout. Slap a healthy dose of getting tbagged by RNGesus into the mix, and you get levels where on mission 1 and 4 you just go to every major area looking for a hit and resetting the level if you don't get at least 2 by the return trip. Missions 2, 3 and 5 just fucking devolve to "run to these spots that never even change real quick, that may or may not take mere seconds depending on if the controls want to work, but don't worry too much about the A Rank because they have ludocrisly generous time limits", making their levels both a slog, and needless 1-2 minute medals/A ranks. Except Route 280. Fuck Route 280, even when you've "mastered" drift mode those karts control like a slip-n'-slide, and fuck that tunnel that always has a car at the end on mission 3. I still to this day could not tell you what the hit box is on the fucking cars, its bigger than them and they're sometimes not solid in the front and can just drive right under you, other times its solid and "kills" you.
Sonic Adventure 2's biggest issue is its trying to be too many damn things at once and it fails at all of them because it has no real focus and just ends up bloated. It wants to be a kart racer, a pet sim, a mech action game, a easter egg hunt game, and a Sonic game, and it can't make any of them work to a level I can call satisfactory. 31 levels you have to beat 5 times each minimum, 3 shitty kart races, 3 boss rushes, and the stupid fucking Chao Garden which is going to take you a inane amount of time to clear thanks to its time mechanics. God fucking help you if you get their stat levels to 99 without abusing the reuse bug or you start with 2 shitty Chao. I wouldn't hate the CG so much if there was something more substantial to it than just being a handful of races and no-input 'karate' to arbitraily tack on another 10 god damn medals at the cost of a required 4+ hours, despite the "challenges" themselves being wrapped up in 30 minutes the second you have two 99'd Chao. But noooo, you got to idle the fucking game and let them evolve because 2 of the medals are arbitrarily locked behind the stupid "hero/dark" system. It really exemplifies the overal bloat issue.
I cannot say at all I enjoyed slogging through it. I'm not 14 anymore and willing to cope with "its not that bad" out of fanboyism or being a poorfag. I genuinely felt like I was fighting the game the entire time more than I was playing it. Its not like it was a skill issue either, when the game would stop fucking fighting me for a bit, I was breezing through A ranks and usually getting it on my first actual completion of a mission. Everything in this game is just a rushed, nearly unplayable mess and it sure as hell doesn't hold up to today. I legit feel that anyone holding it above other 3D Sonic games is straight up being dishonest or under some intense delusion that this game isn't objectively a hot steaming dump on the side of the road.
In 2001 when it was slim pickings on decent 3D platformers, sure, it could get a pass, but then I think of Mario 64 and Sunshine, both games released around the same time (I'm aware Sunshine was '03 but it was still the same console generation). I fucking hate speedrunners and their dick sucking of these games, but objectively speaking these games hold up multitudes better than any Sonic game from the Adventure era to Colors. Hell even Adventure holds up better than 2 despite having most of the same issues, just by virtue of not being bloated to the fucking teeth (an argument can be made for DX and those stupid fucking missions however). I just don't know how many more ways I can say that this game is objectively garbage. You literally can't argue subjectivity with this game because the Mario games at the time from a polish and design perspective make it look like and feel like god damn Croc or Bubsy by comparison. Other 3D Sonics have their issues, but if you're gonna seriously tell me SA2 is better put together than Frontiers, Colors or even fucking Unleashed even with its shitty werehog sections, I have to seriously wonder what you're on.