I'm actually not that worried about them secretly doing something malicious (although that's also a valid concern) but rather in the "given enough eyeballs, all bugs are shallow" and the general risk factors (MikroTik going bankrupt and we're left with obsolete bug-ridden hardware).

Sadly given the large amount of software that's closed source there I don't see them working towards open sourcing any of it as, even technically, it would be a massive effort.