I could trust them more than random custodial or closed-source web wallet out there, but generally using web wallet isn't good idea. Besides, i feel they offer too many services these days even though i hear complain about poor support for Linux.
Is Protonmail still popular? I remember that there was a boom in 2016 and almost every privacy-oriented guy was using it. Is the situation still the same? I haven't seen a proton mail address recently.
I also don't know how popular is it these days. But it's still frequently recommend to people who want email with better privacy (usually better than gmail).
Since their lowest tier paid plan now has custom domains allowed:
https://proton.me/mail/pricing you don't know if the address is proton or not.
davef@davef.com could be at gmail or proton, unless you look at the MX records or the headers of something sent to you you will not know.
As for the web wallet, it's the same as any hot wallet on your phone, treat it as cash you are able to loose with not worrying about it any you will be fine.
Everyone will have a different pain point about getting hacked / loosing funds.
-Dave