Fastmail has contributed a huge amount to the open source community, also include Calendar support (Migadu does not) and has a fantastic web interface (where Migadu is extremely lacking).
I'm fairly sure that Migadu also supports caldav, although I don't think there's any UI for it on the web. I think there's pretty much an expectation that most people using Migadu are bringing their own clients rather than using the web one anyways.
If I really wanted a different web UI I could still just run my own and sync using IMAP.
Their service is excellent and cheap. I highly recommend them.
I've been using Posteo for years, and I have also been very happy with their service.
And yes, the convenience is great, until 1) you stop paying, 2) they go out of business, or 3) you are interested in music they don't have licenses to.
As contentous as the relationships are between spotify-alike services and the music rightsholders, I don't believe that they are a very good long term investment. I almost expect to see the music space to move towards where the TV space is now - each publisher running their own streaming service.