I have yet to find a free contenteditable-based WYSIWYG editor that works properly with the Korean IME in recent versions of iOS. Duplicate characters everywhere. Even the Enter key sometimes works and sometimes doesn't, depending on what character is immediately before the line break. Froala is better, thanks to a Korean guy who helped develop a fix for them despite the fact that it's not even FOSS.
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/