What's missing for Linux? I use Linux + Proton & haven't noticed anything missing?
Well, on second thought there is no Linux GUI version of their VPN is there? I prefer terminal apps though, so this never bothered me. But I could definitely see that as an annoyance to others.
There have been ads on Kindle for a very long time. You can optionally pay an extra $15-$20 to remove the ads though.