Can't speak much to the current stuff, and care even less, but I will also note their licensing and general approach to code openness is rather byzantine (possibly by necessity as many of the "cores" are emulators that were built before GPL got big or purposely eschewed it*) which can get very tricky to navigate. This served to rub quite a few the wrong way and start things off badly, and continue it badly too.
There are some egos involved on every side as well but I do also have to ask the question many times of "what has retroarch ever done for us" and the answer is usually not a great deal -- at best some previously well known emulator devs made a plugin or two for effectively the original emulator (now called a core if we are to adopt such parlances), and they had some fun with predictive input. As far as making emulation more efficient, more accurate, making a frontend that those wanting a debugging emulator can play with (debuggers for non Nintendo stuff tends to be a bit wanting, and if you compare it to PC... oh dear), harmonising savestates and directories (especially for older efforts that might be pre the big push to have everything in my documents rather than program files/operating directory)... yeah not the most on that front.
*not necessarily related but MAME (MESS was a somewhat different thing but I will count it in this) for the longest time was not GPL and purposely did things to frustrate various cabinet makers (ostensibly for profit types but it caught others in crossfire). Similar problems and friction with retroarch, ports thereof, spinoffs and remixes has also happened (the Switch stuff but one thing here, though that is its own can of worms full of misunderstandings, hurt feelings, misdirections, dramas and traumas).
From where I sit then individual emulators still provide me with the best experience. A nice frontend would be nice, better yet a frontend that serves to allow me to have effectively the same debugger on every system without having to learn new ones, one that might also be pretty nice for ROM storage, and maybe blows through some of the cruft in older setups (though older setups maybe still representing the pinnacle of things), sorts controller fun... would be nice, and one that also served to advance the cause of emulation that might have stalled out in some places or otherwise fragmented into a million forks with different goals while it was at it, maybe also porting to various other operating systems and devices, would be golden. However seems to not be so pending that day I am probably in the utterly ambivalent camp.
