Their founder supports Nazis.
First I thought it must be the usual Israel-related dogwhistle, but nope, it's legitimate and related to Swedish far-right Örebropartiet [1].
(Next time, might be worth to add a source yourself to prevent downvotes)
[1] https://www.reddit.com/r/ProtonMail/comments/1uivm45/mullvad...
Pretty unapologetically, too. He's still doing it and will keep doing it the more you give him money.