Most containers don’t package DB programs. Precisely so you don’t have to run 10 different database programs. You can have one Postgres container or whatever.

Well, that's not the case of the official Nextcloud image: https://hub.docker.com/_/nextcloud (it defaults to sqlite which might as well be the reason of so many complaints), and the point about services duplication still holds: https://github.com/docker-library/repo-info/tree/master/repos/nextcloud

You can typically configure the software in a docker container just as much as you could if you installed it on your host OS…

True, but how large do you estimate the intersection of "users using docker by default because it's convenient" and "users using docker and having the knowledge and putting the effort to fine-tune each and every container, optimizing/rebuilding/recomposing images as needed"?

I'm not saying it's not feasible, I'm saying that nextcloud's packaging can be quite tricky due to the breadth of its scope, and by the time you've given yourself fair chances for success, you've already thrown away most of the convenience docker brings.