Main one is searching. I tried it and was unable to search inside message content, only message subjects.

Last time I checked, IMAP protocol was not available, which requires installing on ProtonMail app.

In order to stay encrypted only subjects can be searched. With the bridge app, you can use imap through a desktop applications. I use thunderbird which also lets you search message content.

I’ve never seen this as an issue.

I understand that, you can't search something in web app if web app stores data in an encrypted format.

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.