These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Ditto for the boss of Space Zone (who appears to be Tatanga from the previous game), who is arguably right behind Wario as the toughest boss.
That One Level: Star Maze. Those stars are going to kill you many, many times.
While it's expected that a final level would be difficult, Wario/Mario's Castle can be absolutely brutal. While the rest of the game (barring Space Zone) is relatively easy, the Castle is a difficulty spike that can be very jarring for first time players. Combine pools of acid/lava, swinging spiked balls, spiked traps, and several jumps that require almost-pixel-perfect placement, and you've got a tough level. On the plus side, the Bunny Ears power-up makes the stage much easier to manage, provided the player is able to keep hold of it.