I avoided Retroarch for the longest time, mainly because I always preferred standalone emulators, and after dipping my toes into Retroarch, I still do.
I don't know if I'd go as far as to say the interface is terrible, but it certainly isn't winning any awards. Simple features, like an FPS count, being hidden behind layers of menus is just bleh. I do like that the interface is optimized for controllers, though, that's pretty cool. On the other hand, trying to set your inputs is a nightmare that's the one part of the UI I would say is genuinely miserable. I mean, do there really have to be like 80 different menus just to change my buttons? I guess you could say that for the rest of Retroarch, but the controller options were what really stood out to me.
Still, it has its merits. Like many others have said, Retroarch covering blind spots for consoles that don't have the most robust standalone emulators is quite nice. I couldn't be bothered to setup a command line emulator like Mednafen (regardless of how easy it may be) just to emulate Sega Saturn, but with Retroarch, it works like a dream. For other consoles like GameCube, PS1/2, GBA, which have had incredible standalone emulators in development (with much better interfaces might I add), in development for years, I'm more than likely gonna stick with those over Retroarch.