For me it's the ease of management and good documentation.

For example, during 6.8 to 6.9 upgrade, there was a major postgresql upgrade.

It is mentioned in the doc https://www.openbsd.org/faq/upgrade69.html (see Special packages at the bottom).

You're redirected to the package README with special instructions on how to setup and upgrade: https://github.com/openbsd/ports/blob/master/databases/postg...

Et voilà, everything is explained.

On debian, if I am not careful, I'll do an upgrade and risk breaking something during a db migration (I'm looking at you MySQL upgrades...).

If you run any (Debian-derived) system for more than 5 years, you will run into plenty of these issues. They're always subtly different (the thing that breaks is not the same), but it will absolutely break somehow.