TLDR: It’s compatible with other copy-left licenses like GPLv3. However, it’s available in multiple languages, which technically makes it more applicable.
I started using it for my own project. If you want a practical example: https://github.com/TimoKats/emmer
The weaker licenses don’t even mention SaaS so they aren’t in conflict with the EUPL there. As such, the EUPL’s copyleft protections should still remain for code published under it, even when used in an MPL project.
I’m imagining it as a pseudo-dual license permission and the EUPL as some sort of Affero-LGPL.
Though SaaS vendors would probably comply maliciously and only send EUPL code snippets back when requesting source code, if they are used in a differently licensed project.
I hope you are right but fear that in practice (has this ever been tested?) you might not be.
See for example this discussion ( note especially comments by ‘MadHatter’ )