Ha ha, you clearly dont have much experience using them both. The retroarch core has the standard mame UI in it, and then the retroarch UI overlayed on top of that, so you have two different UI fighting with each other for what UI is in charge, this mostly manifests in controller issues with mame trying to use its settings, and retroarch trying to use its settings.
Wow, i have not heard such rubbish in a long long time, that is simply a completely incorrect statement, i suggest you do more research, as this statement as well as your "i want the most accurate CRT shader" question tells me you really do not know what you are talking about here.
Not true either, yes they are mapped, but for "general" they are not mapped on a game basis, now you try remapping those as the "general" can be a awful setup for some games, and you will see the control issues i was referring too.
As is standalone mame, put your roms in the mame/roms folder, and load a game from its UI, hardly tricky stuff.
Simplified is not always better, in fact far from it, options are good, how you can say that a emulator that REMOVES features is better than one that doesn't, boggles my mind.