This is one of the things I haven’t seen much talk about when it comes to proton (or any other services that use E2EE and stores thing in the cloud).
One of the major downside of E2EE is inability to search on the server side. I think this is not a problem when it comes to client side. This might be a big issue for people who have to search through a lot of stuff(notes/emails/calendar).
[rant]Unfortunately, PG fail to mention this. Just recommend having it for everyone.
This article (I don’t have an affiliation with their products) does a pretty good job of explaining E2EE.