It's standards based and interoperable. I'm using Thunderbird on desktop and JTX board + DavX on Android. I like the freedom to chose what I want to use.
I don't understand the usecase for tuta and proton. Their "automatic encryption" only works for people also with those providers if I am not mistaken, otherwise it just uses the same method as PGP (password that you have to send the recipient).
It seems to me that for 99.9% of emails going through the account, it is not any more secure that other providers like mailbox, posteo, nubo, etc...