No IMAP, doesn't support open standards, proprietary encryption, no compatability with 3rd party apps/mail clients. You get forced to use their app and have none of your mails locally. Also, since their encryption is proprietary, they only support password less encryption between tuta users, so 99% of users will only ever have encrypted mail with the tuta advertisements/announcement (though they market the "encrypted" angle extremely hard.

Not to mention all of their apps are exclusively web and electron and perform badly with a very very barebones user interface and options. Also the search is pretty bad. All things that would be fixed if they support open standards. I really dislike using their apps and interface with my account.