Korean is different from Japanese, but as far as IME complexity goes, a Hangul IME should be extremely simple to develop compared to a Japanese IME. (The Hangul part of a Korean IME doesn't need any pop-ups like a Japanese IME does. As far as UI requirements go, the Hangul part of Korean IME can be as UIless as a Vietnamese Telex IME.) That e.g. on Ubuntu 18.04 the Korean IME is broken is not due to anything intrinsic to the writing system.