For some reason I have it in the back of my mind that they were at one point accused of being a honeypot for US intelligence because of their association with MIT. Probably complete BS, but maybe not. Are they as open source as they claim to be? Looks like they’re on github. F-Droid seems to think they have some Google libraries or whatever that they use.
ProtonMail users, how do you like/dislike it?
On the open/closed source issue: how do they stack up against things like Tuta and Signal? The latter I heard was not actually completely open source.