GM does have a lot of interesting ASM, but unfortunately the game is really meant to be almost Kaizo difficulty as you get further in. Presumably because over in Japan, there's no real need to be 'fair' in ROM hack level design as much as just show off your ASM skills.
But the final castle in the latest demo (castle 2) is really poorly coded (I've had the level glitch up on me before and stuff looking like glitched tiles/static and what not appear), and the first one, while interesting has one of the least fair boss battles ever.
Oh, and some notes about the hard to find hacks mentioned.
Ore World 1 is horrifically designed. It's average ASM, with some custom bosses, but the level design is... well, less than Youtube quality.
Ore World 2 is really quite basic level design with decent boss battles, but gets very, very repetitive, and has some extremely hard custom boss fights (oddly enough, the first boss is harder than the Nightmare boss from world 2, and the third or so boss, I think Kabula is harder than King K Rool from the end of the third world).
Scarlet Devil Mario is repetitive, VIP 5 is a tad overrated in my opinion simply because most of the level design doesn't work that well, and the stuff seen on Youtube is all right at the end of the game.
But yeah, in general, most Japanese hacks seem to have fairly mediocre level design I'm afraid.
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As for Mario's Nightmare Quest? Well, it's currently on Fusion Gameworks, ROM Hacking.net or the GCN at the moment.