The big problem these days seems to be absolutely lousy user documentation. It took me probably 3 full hours of googling forums to figure out the magic combination of file locations, names, command line switches and file formats to get some turbo CD games I have working with RetroArch...and I've never been able to get sega CD games working at all...I actually managed to do it with a real MAME download instead -- which had its own association google hunt all over the internet to get up and running.
There's absolutely zero documentation in any way explaining how to setup the rube goldberg machine that makes these more complicated systems work.
It used to be something like
1) put bios in this folder and use the gui to map the config to the file
2) point to the iso or cue or whatever
3) run emulator
I feel like modern emulators are getting more and more opaque in how to set them up.
Dolphin enjoys major support from the competitive gaming scene, specifically Smash Bros. Melee players.
I'm sure it also helps that it emulates two consoles in one.
In some cases, that's all that's needed. higan (formerly bsnes) is essentially perfect as a SNES emulation engine, and openemu makes it really easy to install and use.
My experience with pretty much every other emulator: extremely byzantine UIs, no quality control to speak of, etc tc.
RetroArch is the hope, but I'm not convinced yet.