Turn speed whilst moving.
If you’re stationary in either game, you turn to any direction instantly and once you accelerate to a certain velocity, the turn speed quickly drops to allow for precision movement.
However, in Sonic Adventure that limited turn speed is not limited enough. The game designers obviously expected it to be a fun mechanic to move 4 feet to the left, then 4 feet to the right roughly seven times per meter per second they are insane.
Mario on the other hand, despite not being as much a physics based platformer, turns much more slowly, making the collecting of coins fell quite natural and satisfying. Sonic on the other hand is blazing along at 100 miles an hour and you are expected to make minute adjustments to your trajectory in order to get the tiny ring hit boxes when just tapping the stick send him careering almost st a right angle. Oh and your area is ten feet wide. No here’s some really confusing platforms and bottomless pit, enjoy!
Then you have the story, yeah I get it, everyone wants the gems.
The spin dash is still useless but Sega managed to create a game where collecting gems/rings and making minor direction adjustments nigh on impossible.
Mario 64 is simply a better made physics game with levels that don’t drag on and controls that facilitate accurate level traversal and with a superior camera.
Sonic team still haven’t figured out that if you are moving fast, minor changes in direction result in major changes in your course.
So they keep trying to make a slalom game but putting crap stages in with bottomless pits and jerky controls.
This is the reason they invented the homing attack. Rather than fix Sonic’s control and camera issues, they just made it like Afterburner but less easy to lock on. This goes for stages where you control robots, now they can hardly move, despite being a super speed woodland creature.
This is why the franchise faded and why Mario games are far more popular than 3d Sonic games these days.
EDIT: If all the games were like this (1:40:40). They would all be good games. That is, if you could control them.