Personally, I think it's unfair and silly to judge ROM hacks separately from other fangames, and it's even sillier to do this because of effort involved, as if these two broad categories properly represent differing difficulty of development.

On the side of ROM hacks, some games are going to be easier to hack than others. Super Mario World, for example, is going to be especially easy in this department, compared to hacking, for example, some random obscure NES game, or even Super Mario Bros, thanks to Lunar Magic.

It's even more varied on the side of other fangames. On the low end, you have games that use Hello's SMB engine or even regular Hello Engine. But even without those, a Game Maker game is a lot easier to develop than a Java or C++ game.

Comparing all of the games, using Hello Engine is going to be much easier than hacking Super Mario Bros to change the levels. I don't think I need to explain why C++ from the ground up is harder than hacking Super Mario World.

So really, ROM hacks should be judged exactly like other fangames, rather than attempting to make it "fair" by segregating them or excluding one.