Sonic 2 is so dang easy. I can definitely see how it'll catch you off guard because you do have to learn the patterns with no room for error, but the patterns aren't difficult and in all, Sonic 2's bosses I feel give more opportunities to hit them than any other 2D Sonic game I can think of (never really played much of Advance).
Mecha Sonic probably isn't a good boss because if left to live, I don't think there's really a way to tell if he's going to spin dash or spin and jump or if spikes will bust out of him or not, so it's pure memorization if you can't beat him quickly. But since he just stands there for a while, you only really need to know the first two or three moves he does (spin dash, flying back and forth and vulnerable, spin and jump), which he does every time. You just jump three or four times on him when he stands there and he's finished in 8 seconds or so.
Dr. Robotnik has plenty of tells, and the thing is, you can hit him in between each step he makes, making short work of him in a way you can't do in Sonic 3 or Knuckles or even Sonic 1. Unless you play with Knuckles -- I'd say he's legitimately a harder boss with him, or at least one with more patience since you can only hit Dr. Robotnik during a few spots.