I don't know about Mario hacks but I'm in agreement to some extent about Mega Man hacks. Granted, I haven't tried any in ages but I remember having similar luck. I don't know how much of it is just from unfamiliarity compared to some of the originals and how much of it is the actual difficulty or even my skill level.
I do remember some of them having cheap design in parts, though. In particular, there was one hack of the first game that put a small hole in the front of Ice Man's boss room. If you walked in, you slid to your death.
Worse, because the boss entrance code had already been activated, you couldn't even move when you respawned. All you could do was stand there at the respawn point and wait for those flying enemies (weren't they like bird bombs or something?) to knock you to your death again and again, because of course there were more holes.
The trick, apparently, was to make sure to jump through the boss gate. Still a pretty cheap trick to pull on a first time player, though.