Personally I think Retroarch is great for GB/GBC/GBA and 8/16-bit emulation. Everything else has superior standalone emulators.
I totally agree. 8 and 16 bits basically.
I also use it to play PSX (has some nice cores there, so, yeah, no need more than that) but not Saturn, N64, or anything greater than that, like Dreamcast or PS2. I mean, I can test those cores for Retroarch, from time to time, but... I prefer other dedicated emulators.
Another exception I do were I also use Retroarch, is to use its 3DO core. Because 3DO emulation is very strange and not that common, so, you better have 2 emulators at least. (The other 3DO emu is Phoenix, a BIZZARE emulator in Russian, which emulates 3DO and Jaguar for some VERY ESOTERIC reason I can't even understand. No, they do not have the same CPUs or architecture).
But even for Atari Jaguar I prefer to use another dedicated emu, BigPEmu, which emulates for the very first time Jaguar CD games, since 1 or 2 years ago: Finally you can play all that crap in your PC. And be thankful, because those Jaguar CD units were always shit and they are simple dying since 20 years ago (at least). So... some exclusive marvelous Jaguar CD games had the real danger to be not playable anymore until that emu appeared and give us joy to everyone of us.
So... only 8bit, 16bit, PSX and 3DO.
Fortunately, it is not the same case of MAME,
were you HAVE to use it, to emulate HUGE QUANTITY of arcades, yes or yes (and we all know MAME, an all its different versions, can be a pain in the ass to config when you want to play a game for the first time). There are few notable arcade exceptions, like Model 2, Model 3, or NAOMI... much better emulated outside MAME.