I personally don't truly consider something emulated until it's in MAME/MESS. The problem with a lot of the one off emulators is that they just have shitty video output.
Take Demul, for example. Yeah, maybe it emulates Naomi games better than MAME does, but it has microstuttering and runs like ass even if you're getting 100% speed.
MAME has no microstuttering. It runs very well.
BSNES has the same problem. Sure, it might be an accurate SNES emulator, but it's a shitty Windows program. I've tried BSNES on at least 8 different computers, and it couldn't run without hitching/audio breakup on any of them even though they all got 100% speed. It also only runs in windowed mode, so it doesn't work with G-Sync.
MAME's the only emulator that really feels bulletproof when you actually play games on it. It's kind of ironic given MAME's goal of being a documentation project; it plays games better than any other emulator I can think of.