Last time I checked, IMAP protocol was not available, which requires installing on ProtonMail app.
I’ve never seen this as an issue.
It's a classical security vs. convenience tradeoff.
I barely use desktop clients for email, except for backup. It's either web app or mobile app, and here Protonmail doesn't work for me.
My use case would be a trusted non-megacorp in non-eyes country with standard security. Something like for example Fastmail based in Switzerland.
Last time I checked, IMAP protocol was not available, which requires installing on ProtonMail app.