The only thing I can really complain about in Super Bonk 2, is the checkpoint system. All other Bonk games have Bonk respawn. It makes it more difficult, for better or worse. Bonk 3 was waaaaaaaaaaay too easy, so challenge is welcome. Still, I found the final boss of Super Bonk 2 frusterating because of not respawning. That's one thing I'd change if we brought it to the Turbob.
I like the checkpoint system. It's a nice change of pace and the game is still pretty easy overall.
Bonk 1's final boss I feel is much, much harder and that's with respawning.
I'd say Super bonk 2 is more challenging, solely because of the pits, and it's harder to get heart pieces. But don't forget, it's the only Bonk with a password system.
Honestly, my only real complaint with Super bonk 2 is the repetitive Castle levels. But in the end, it really feels like a proper Bonk 3.