Your data, your rules Proton provides easy-to-use encrypted email, calendar, file storage, VPN, and much more, built on the principle of your data, your rules.
Proton lets me control my data without surveillance. Migrating my mail was the keystone, but I will likely use more of their products. They have built viable alternatives to many of things I use Nextcloud for, like cloud storage and calendar.
How seamless is using Proton across mobile and desktop?
Proton Mail works well for me on iOS, MacOS, and Ubuntu (Linux) desktop. There can be little lags but everything syncs up.
Nextcloud
ExpressVPN