Sega Genesis looks horrid, SNES looks even more horrid, they play uber slow and the sound quality is way off.
Only Gameboy, and NES/Famicom actually play well. And no decent Atari 2600 or any other model exist yet, so that's out of the question.
So I'm guessing SMS, TG-16, NeoGeo, 7800 and MAME aren't that great either?
You would be accurate in that assessment.
But for NES/Famicom and Gameboy it does a wonderful job.
Edit - Download a program called BootDreams to make a disk formatted correctly for Dreamcast.