Drive has no Linux client, Photos is extremely barebones and locks you basically in, as there is no export function.
Pass still has no proper SimpleLogin integration, no credit card support and UX wise is the browser extension pretty bad. Funny enough, years after launch you still can’t auto fill on Reddit.
The only thing I don’t like about Mail is that you still have to create reverse aliases through SimpleLogin. Better integration would be great.
Contacts still don’t sync to you local mobile contacts. Which means you either do it manually or you have to keep two sets updated.
Calendar is good too, I’ve heard it has no offline support though. Although I haven’t verified that.
Last thing I would like to see is notification support without Play Services.
Some of those things might be super unimportant to some, but for me it makes the use of their stuff unnecessary cumbersome. Especially if you consider that those are all Proton products and should work together well.
My by far biggest problem is their communication and general development speed though. Stuff like contact sync has been requested for 5(?) years now but there hasn’t been so much as a “we’re working on it”.
It feels to me they come out with new products all the time, like the document editor now, without addressing the little things that would make their ecosystem great.
Anyway, long ramble. But I appreciate that you asked for more details without insulting me.
❤️