@ei8fdb These apps are mostly unmodified versions of ones you would get from the developers of said apps. You shouldn't have any issues exporting a DB, moving the DB and files over to a new instance (outside of Cloudron) and grabbing data you need.
Speaking of Nextcloud specifically, you can move the DB and files + the config with little issue if you needed to, but I don't think you'd want to after moving to Cloudron, it's that good.
I've been using it since version 3.x and I gotta say I've never used a system like it - It rocks. I've tried Mailcow+Portainer, Mailinabox, I've even self-hosted with just straight up ubuntu + Apache (manually installing and managing), but Cloudron takes the cake. We even have a LAMP app that can host PHP, nodejs, etc apps that aren't otherwise supported.