While I agree with you that the security in UnRAID seems pretty weak at default settings, your router admin page should not be accessible from the outside if you configure it correctly and keep it up to date.
You highlight a big problem though, default settings in all these docker containers we pull, and I think that boils down to the individual user and the software being used. Your friend is tech savvy enough to setup his own OMV on UnRAID so he should definitely be techy enough to know to change the default admin password. And the software should be made in such a way that default passwords are a major error event that fires warnings everytime you log in to it.
2FA is in my opinion a complementary security feature that should not keep a software secure on its own.
But I hope some big steps are taken in regards to security by the UnRAID team going forwards. I'm still on my trial period with 12 days left and I really love UnRAID but I keep being scared on some security defaults (SSH enabled with password even though the keys are generated and stored on flash, no simple switch in UI to disable PW logons, why???). Root as default user, major functionality put in the hands of the community (Fix Common Problems etc) which is a huge attack surface because I guess these plugins in UnRAID run as root? It only takes one big community addon to be hit and a lot of servers will be infected, and I guess UnRAIDs stance on this issue will be something along the lines of "you used community addons on your own risk", which is true.
Sorry if I'm ranting in an somewhat unrelated thread as this post is more about general security on UnRAID.