You're right about MAMEUI and the lacks of some new options introduced in MAME core in these years. Though it seems Developers don't (want?) to get the point: Internal MAME UI is not bad but, due to the "obsession" of being universally compatible with all OSes on the earth, it misses a fundamental thing: the word "Graphic" in the acronym GUI. Here is an example of what was my GUI code. When the internal UI of MAME will be able to produce something similar, maybe a lot users will stop to troll it.
P.S: Did you noticed that old-CRT monitor style in the icons? :P