[zogrodea]

Android is pretty open as far as I know. You can even run Linux distributions on it with Termux - although its future is looking unstable with Android's new OS versions changing the services it depends on.

I think (guessing) the main thing an OSS advocate might take issue with is that decisions are made by corporate bodies that might not benefit end-users in every case. For example, the aforementioned Termux issue and how Google recently deprecated the open source Messages and Dialler apps on Android, with vendors like Samsung giving proprietary alternatives.

That seems to me to be an issue with some other open source projects too (people complain about GNOME for example not listening to its users) but it possibly happens there on a lower scale since Android's a bigger project (likely most people in the world have seen or held or used an Android phone once in their lives).