I like their mission. I used them for a while and the service was great, but I got annoyed at not being able to use any other client than theirs. I wanted to use Thunderbird, but they don't support IMAP for encryption reasons. I switched to Fastmail and it's been nice, I like that they're fixing the shortcomings of email by pushing for new standards like jmap. I'd probably have stuck with Tuta if they had a plan for a new standard that let them be privacy oriented and also let me use Thunderbird.
Yep, it is a bit too big walled garden at the moment. Since it is E2EE almost completely, it is harder to make alternative clients. But still they should offer some specification.
