The main reason why I switched from Fastmail to Proton is that you don't have to rely on their apps, even though they're far better designed and much more powerful. Example: Proton's Android application still does not support conversation/thread view but you can't even use IMAP/POP3 because of the encryption.

I also believe that the benefit of Proton's client-side encryption is extremely limited because most of your contacts aren't on Proton so they'll keep a plain version of your emails. So you're only protected from Proton looking at your content (while you can pay them not to) or from the police asking Proton to share your emails. But Proton was recently forced to give on of their users' account because the French police abused the anti-terrorism laws (through Europol) even though the guy was only suspected of illegally occupying a building in Paris during climate protests.

So you end up with lots of small disadvantages (running Proton's bridge for emails, or having to rely on their poor mobile apps) for little protection.

Edit: also, when I trialed Fastmail, I discovered that the Protonmail mobile app fails at manual forward to my Fastmail email address (on a custom domain). I've reported the issue 5 times (with all the info to reproduce), reinstalled the application and tested on other devices bu they don't even recognize the issue. They do reply but I still can't email myself due to a bug in their Android apps (both stable and beta versions). Quite frustrating.