It is more the UI/list aspect. take e.g pgadmin and click open the list of users. I have a db with maybe 1000 users and even with that low number it gets annoying to navigate. Compare with e.g. LDAP tools that allow to organize users in a tree.
Now 1000 is still survivable, but the experience made it clear to me that one doesn't want ones whole corporation in 1 long list.
To be clear: This is not a problem of the postgres ecosystem, which has very high quality standards and deserves much praise. But you get a feel that it wasn't intended to be used that way.
Do you have an example of such tool?
> as it assumes it can load all rules in memory and show them in a list
Which tool has trouble loading millions of users in memory? This should be a few megabytes of RAM usage at most