Im using proton mail for 8 months now, there are some considerations:

On desktop you can use the mail bridge, so you can use any mail client you want. But that means running an extra piece of software.

On mobile you’re stuck with the proton mail app, no options there.

You cannot in a supported way use it with other apps or software that needs to access your mailbox for reading your or any other mailbox (for automations or automatically importing stuff etc.)

For business account users there is a way to send email via other apps or devices (think notifications, scan to email etc)

This all has to do with how your mailbox is stored encrypted on their servers.

The web mail client is decent though, if that’s your thing.

Overall I’m happy with the switch