I don't know if they've finally resolved it, but BARs biggest issue last time I looked at it (which has been a bit!) was that they (spring) implemented the game with deterministic, lockstep multiplayer (good!) but used native floats, compiler fastmath flags, and stdlib math functions throughout. So you'd desync instantly playing against anyone whose game was built with different compiler flags, or Windows x86 vs ARM OS X, or different versions of libc, etc, etc.

That said, BAR was great fun when I tried it and very well done. Even has a rapidly growing youtube community.