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.
They aren't bad at all but be warned that you shouldn't use them for throwaway addresses that are lightly used. They will delete your account if there hasn't been activity for 6 months.
Sounds like a lot of time, but surprising how quickly that time runs out.
Something to be aware of up front.
similar to protonmail, european, came out of the post-snowden shift towards e2e services