I use them too, they have some good stuff, but are pretty behind in usability.

• No Linux file sync client, making the drive pretty useless for me.
• Android file sync is only bloody images for some reasons
• The android proton Mail Mobile app is missing some features, like select all
• You can't use any other app than the official one on android (If you don't use something special to access their server)
• You need the proton bridge to be able to access their servers on regular computers. This only works for Mail, so no calendar or contacts sync. You can only host the proton bridge on localhost, meaning that automating SMTP for a server with containers or vms just isn't possible. Every host needs to run the damn bridge.(proper SMTP is apparently available for businesses)
• Can't sync the contacts to my phone (only upload my contacts)

I just wish they did usability a bit better, it can't be so hard. There is probably more that I didn't think of right now. Don't get me wrong, they have big potential.

• They do pgp (yay)
• They offer hosting mail for your domain (just set DNS to their values, it just works and there haven't been any problems with that part.)
• Pretty pro open source
• Seems like the best alternative we can get to megacorp data stealer clouds, besides Selfhosting perhaps
• I've never had any downtime, which is why I switched from Selfhosting Mail to proton
• I'm sure they have much better security than my selfhosted mailcow