In theory alone, yes - commercial titles expect 100% control on both CPU and coprocessor (IOP) side, so any additions (like OPL drivers, but could also be standalone GSM, action replay, etc) come with a risk of conflicts, and in particular OPL needs a lot of code in proportion to the 2 MB IOP side memory, but the whole USB stack + FAT16/32 is smaller than expansion port + network adapter + tcp/ip + SMB
Speed usually doesn't matter for making or breaking something as (decent) disc software is OK with slowdowns (dirty/scratched discs)
That's much harder, in great part because there's nothing close to a complete compatibility list (implementations evolve, for better and worse, with OPL updates so individual lines in lists get outdated) and, secondarily, because all sorts of individual interfaces/servers can be used in a single category
I think, but am not sure, that firewire and SD adapters are both BDM implementations?