I can't actively say too much about the security of Protonmail.

However, I do know a lot of people that know the founder (Andy Yen) socially and they all uniformly think he's a scumbag. (Doing things like going to car shows with a junker saying that his nice <insert car here> is in the shop, other generally sketchy/manipulative actions.)

Overall, I'd say Protonmail is good at being a hype machine and while there may be some technical correctness in what they're doing, it's not a business you'd want to rely on.