Personally, I maintain a separate compose file for each “end-user service.”
So, e.g. Nextcloud has its own compose file, including a Nextcloud-specific redis and database. That way, if I need to migrate my Nextcloud instance to a different server, I don’t have to disentangle its database from e.g. the database that my FreshRSS installation is using.
This also provides some benefits in terms of security. If one of my services turns out to have a SQL injection vulnerability, keeping its data in a separate database means the systemic damage will remain compartmentalized.
The compute overhead for most of these “backing databases” is negligible. I think it’s worth the overhead.
Well, this is certain to have absolutely no worrisome sequelae.