You're also locking yourself into an ecosystem without open apis. If you want mail on your phone you have to use their client, if you want it on the computer you need to install the protonmail bridge (which seems to restart itself and overlay over the top of whatever you're doing)
The apps don't seem to behave well on MacOS either, something about how the windows are designed means that they won't show up in the Mac cmd+tab switcher.
All this means that their security model is irrelevant to me, if I can't do the basic things that I need to do.