I haven't tried mailinabox.email, but I've run a mailcow-dockerized instance for about 4 months.

It took about 1 day to set it all up, where most of the time was spent waiting for DNS changes and for Microsoft to de-list my IP from their blacklist (I probably got a bad IP from DigitalOcean).

After that I have logged in to the instance every two weeks to update the machine, haven't had any problems as of yet.

The uptime shouldn't be any problem, as the other mail providers should retry sending their mail, it's even mentioned in the RFC: https://tools.ietf.org/html/rfc5321#section-4.5.4

Hopefully I haven't missed any mail when updating mailcow-dockerized, I'll never know, hehe.

Really easy and a great experience, compared to doing it from scratch.