Sometimes, it hangs and never shows the Inbox or takes a very long time to show your Inbox (several minutes). You have to clear your app cache when this happens and sign in again. You also don’t get full access to settings. You cannot go in and add a new alias from the mobile app, or change payment options.
Fastmail is okay. My gripe is you can’t use biometric security on Android and when you set custom color schemes on the web client, the app disregards them.
I use both. I used Fastmail since college when they were the big thing and they support and develop FOSS software. I migrated to ProtonMail out of curiosity after more than a decade, but when they turned over the IP address of a fucking CLIMATE ACTIVIST to police, I decided to halt that process and later on I renewed my domain with Fastmail out of convenience (other things that irked me were were the Proton Mail bridge didn’t work on OpenBSD and I couldn’t use it with alpine or K9 mail at the time). I keep ProtonMail for occasional registration/verification email that doesn’t make it to Fastmail but I’m not under any illusion about protection under Swiss law.
I was so entrenched in Fastmail that I stayed with them because the annoyances with ProtonMail service didn’t outweigh the benefits or price increase. If you are starting from scratch, I would probably go ProtonMail.