They have a very common (happens in nearly every JS-based editior) yet very ugly bug. They're registering right Alt button as a standard shortcut button (very stupid and useless, yet again very common pratice). This provents non-English users from entering national characters.
EDIT: For example: I can't enter Polish letter "ł", because standard, system-wide combination for entering it (Right Alt + l) turns out to be enabling / disabling upper script in Redactor. I can't enter "ż" letter, because Right Alt + z turn out to act like Ctrl + z and thus is executing Undo operation instead.