I don't recall the exact specifics, but there are limitations. One I think is the support for certain bands. There is an issue with VOLTE requirements, an example you can read about here, although I do not understand what it is. (Sorry for Gitlab lnk.) I've seen it show up on phones where the 4G and 5G indicators are located.

Not sure about graphene. I don't think anyone can use most of the Linux OSs here. Graphene seems to do well but you have to buy a Google phone. I use e/OS and as far as I know as of Dec 2025 only Murena and Mint Mobile will support it in the US.