Someone was asking that long time ago, but I don't remember who. Basically, what is BUG.
Between the time saved by a CAR merger, and the time lost by the BUG merger (as far as graphics time)... might break even and end up with a superior product...I have to check it out, but I guarantee you that BUG also slows down graphics. There are people that are having hard time playing RFCE with the current set of graphics, so this may kill them altogether.
Merging RFCE and whatever other mod, would require file by file copy/paste of things for Python and perhaps C++. This is hard and very time consuming, thus it is a low priority.
I think CAR mainly optimises C++ code and caches Python values. That's why it works so well in the vanilla game where there is little Python. The more Python you add, the less important C++ is, and the more often you need to check the Python values.