Are you denying that is the case? because I'm pretty sure the majority of the dev team would agree with the statement.
It's seen no *real* advancements in a long time, it needs rewriting using proper modern code, not win9x era stuff. You ended up even having to hack out the memory initialization code from the entire emulator just to keep some UI code happy not long ago (and initialization is important - that's why the GCC6 bug in the latest version is so bad)
I'm not criticizing MAMEUI because I don't like the idea of MAMEUI, I actually think it's rather important to have a proper fully native integrated Windows version, I'm criticizing it because the one we have is just plain awful at this point in time (that didn't use to be the case, but things have moved on, significantly)