From memory, there are no bulk actions, so if you want to say select all emails and delete you have to send thousands of requests. If you want to rename a folder you have to send a request for every email in the folder. There is no way to set up filters that run server side, there is no way to get push notifications.
And probably a million other things that don’t hold up today.
Sending those thousands of requests is something your mail client does for you. Deleting 5000 emails takes a few minutes, but how often do you do that? I can select a bunch of emails in Thunderbird and just do stuff with it just fine.
For server side filters I just set them up in Fastmail using the web UI. That's the type of action I do once or twice a year, so totally OK to hop on over to the web app for just that.
I have no idea what you mean by 'push notifications'. I have Thunderbird open on my desktop, and it shows me when there is email. I have K9 on my smartphone, and it shows me when there is email (I don't have it set up to display notifications, but that seems possible). That's basically all I need to do email.
Deletion is done by marking messages with deleted flag and then expunge to delete flagged messages. AIU rename exists: https://datatracker.ietf.org/doc/html/rfc3501#section-6.3.5
Push notifications: https://github.com/freswa/dovecot-xaps-plugin