I can attest this for photos.

Google Photos is a third-party app, and it syncs photos a few magnitudes faster than Apple Photos.

I never know in which state Apple Photos are at any given moment: is it syncing? is it stuck? why?

Apple Photos manages syncing based on things like network connection and especially battery life. If your battery is below a certain level you have to manually resume syncing. That might be part of the issue.