Personally, and I have no relation to OP, there was no compelling security advantage for me. Email has no security guarantees unless you use PGP and I don’t know anyone who uses PGP. If someone wants to read my at rest mail they are going to compel me to hand over my keys anyway. And I think the best security policy when it comes to file services is: don’t.

And on top of that I need to back up my email to my offline storage and doing that with their proprietary stuff is a pain.

In the end I just moved to Fastmail and use it as a simple IMAP/SMTP service. Emails I don’t need any more are archived to offline folders in TB.

My entire public cloud exposure is literally one imap mailbox (with 11 emails in it) and 1 static html file in fastmail’s public web service infra. Oh and separate DNS/domain provider.