A big reason why multithreaded UI doesn't work is that the platform code (win32, cocoa) generally has poor support for interacting with OS objects from multiple threads. For instance, NSView instances can only be operated on from the main thread (https://developer.apple.com/library/mac/documentation/Cocoa/...).

Handling OpenGL context access from multiple threads is terrible, too. I'm quite excited about the additional threading niceties that we're getting in Vulkan, which should allow separate threads to re-render parts of the UI and then send the command buffers back to the main thread.