I'm moving away from gmail in favour of protonmail, so far I've experienced no issues.

I've emailed my contacts informing of the switch, and I'm passively migrating important accounts/services; in fact, this forced me to consider which services are actually important and worth receiving e-mail from. If I go a couple of months without getting e-mails from some service and nothing bad comes from it, then I probably won't ever read it anyway and it's not important.

As soon as their mobile app is better, I'll likely switch to their paid service, although a private calendar and a corresponding quality app would make the 5€/month seem like better value for money.