I used to think level 10 was always the level that the barrels would be less controllable, and that was sort of a gift, because to DK programmers, the controllable barrels where actually meant to make the game harder, and maybe there was a "relief" level where things eased off a bit.
But, that could just be randomness.
To me, the conveyors are the ones that get weird to me mid and later in the game. Always have to be fearful of that "fireball in the belly" when getting the bottom hammer. That's the one situation that gets me once every now and then that I've yet to find avoidance from. (Back jumping the hammer away from the fireball is the best move, but if there;s a pie behind you you're screwed).