I have been testing extensively with a set of >800 .pak and except 8, all of then loaded and work like a charm. I have not fully played from beginning to end all of them to say if in stage 2.3 there is a glitch in a lamp or something like this, but with the same set of games in Recalbox, for...
No idea, but I guess it will be because, since it aims to open properly as many pak as possible, there will be some games needing these builds. For example, this one requires a quite recent build: fgames9000.itch.io -> streets-of-rage-x
I have found that there is already a OpenBOR libretro core for RetroArch
github -> retrodiv -> AnyBOR-libretro
It uses internally many OpenBOR engines, so maximum compatibility with most of the games through the different versions