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


I can only answer the first question:
The great thing about the EUPL is: Its terms prevail if the other license does not have conflicting provisions. Any code licensed under EUPL will keep the SaaS restrictions:
https://interoperable-europe.ec.europa.eu/collection/eupl/matrix-eupl-compatible-open-source-licences