It is true that working with IME is hard, even though it is kind of supported by operating system and browser.

As much as it is doable for standalone editing, it is a nightmare for collaborative editing, where content changes all the time - not only at the place where you type. Each content change may cause composition (IME) to break. That's how the browser works.

This is one of the reasons why "real-time collaborative editing" solutions often implement "block limiting" -- one block/paragraph can be edited only by one user at the same time.

Enabling IME will be one of the toughest goals for CKE5's collaborative editing solution - Letters: https://ckeditor.com/letters/