Well migadu, like Proton technologies, is based in Switzerland.

A lot of it comes down to reading over a company's privacy policy (if you can't understand it / its really long, that's a red flag), where they are located, and -- important -- if its free or paid.

Here's the write-up that convinced me to switch: https://drewdevault.com/2020/06/19/Mail-service-provider-rec...

Protonmail is mentioned in the footnotes.