Not sure what you mean, but @JoshData hasn’t put any clock on MIAB, and it has a strong, if small, community around it. Since MIAB is designed to be easily audited and built on preexisting software packages, there’s no real issue using it over the long term.
MIAB isn’t mail software itself. It relies on Dovecot and Postfix which it will install and manage for you. You can use any migration toolchain that supports POP/IMAP, or any software that can read or process Maildir.
There are some individuals reporting in on the forum that they’ve had their companies running on MIAB with under 100 mailboxes (see Is Mail-in-a-Box Enterprise ready?). Note however that since MIAB relies on Postfix and Dovecot, there’s no hard limit to the number of mailboxes that can be supported other than those limits imposed by your hardware resources.