Bumped into Johnny The Torpedo Shark Robot Thing first of all, but couldn’t beat him. He gets far too far ahead right away, so launching stuff at him doesn’t work (they don’t hit him), and I can’t boost to catch up as he’s too fast. So I gave up on him.
Instead, I explored another couple of islands, and then stumbled across Sky Babylon, which was actually pretty difficult with lots of electro-floor insta-deaths and precision jumping. I found a problem with the jumping too: When you move right, the “viewport” of the screen adjusts to move Sonic further to one side of the screen. This is to allow you to see a bit further ahead when moving at speed. However, if you jump just a little to the right, say, then stop, Sonic appears to jump up, right, then left on his own, before moving to the right again as you stop. Of course, he doesn’t, but it makes difficult jumps harder than they need to be as it feels like you’re been blown backwards.
But finished it in the end, and the boss was pretty ace too, so it wasn’t all bad.