They weren't incredibly new, Mario 64 and Ocarina of Time had been out for years. Goldeneye existed and F-Zero did as well.

Most genres had translated fairly well to 3D by that point.

Sonic Adventure overstuffed itself with unnecessary characters, mechanics, and didn't focus on making the minute to minute gameplay particularly fun.

None of the issues with SA1 or 2 are from 3D growing pains, they all stem from Sega never nailing down the core Sonic gameplay loop in 3D and skipping straight to tacking on extra gimmicks instead.

To this day, they still haven't done that, despite their most well-received 3D Sonic games also being their most stripped back in Generations and Colors. Colors also has gimmicks but at least they are all directly in the service of making the game fun and expanding Sonic's moveset.