I personally run into very few bugs with Sonic Adventure. Are the controls not very tight? Yes. Are the graphics and animations silly? Yes. Is the world vast, glorious, and filled with so many things to interact with? Absolutely. This is what truly separates Sonic Adventure from the bad Sonic games that may have these same problems.
That actually makes a whole lot of sense. Perhaps this might be some sort of evidence of the two worlds? What of the action stage entrances? Are they a gate of some sort as well?