There is a special need on this server to use a lot of network shares assigned to drive letters. It runs a really old (pre-2k) HQM software, thus join/subst/mount solutions don't work.
The situation were a little bit better, if I could use A: and B: as network shares.
My question were, doing so has some non-trivial fallback, or it can be done without any problem (although it is uncommon)?
There's no reason not to use A: and B: They're not used these days for more legacy reasons in the way that admins were used to seeing them as floppy drives so they aren't used out of habit.
I have a share mapped to a: on my server and it's never caused an issue.