I am no programmer so I may well be completely wrong here. But I could imagine that the main problem with offering complete encryption on a cloud-based service like Evernote may be the search / indexing function. To my understanding, the search mechanism needs to access ALL notes if you do a global search. So to find a specific word you would have to either download every single note and decrypt it, so the search term and content could be compared for a possible match. Yes, I know, this is simplified because index tables are searched, not the actual notes. But the problem is basically the same. I'm not completely sure where the search in Evernote is done; offline with a search mechanism on your device (I doubt it, think of Smartphones) or online on the servers. If the latter, I don't know how it could possibly work because all notes would have to get decrypted on the server first, which has to be impossible if you want an end-to-end encryption.
Simply put, to find a note, the server has to read it. But the purpose of encryption is that the server can not read the content. How to solve this?
As far as my limited understanding goes, full encryption would be possible but it would break the search functionality.
OneNote btw is facing the same problem. Yes, it can encrypt complete sections (which, in case you don't know, is the middle hierarchy level between notebook and page) but the search function is always excluding encrypted sections.
I'd love an expert, maybe even an Evernote engineer, to drop a few words on this and hopefully confound all my theories.