The single most important factor when it comes to UI is text rendering. Immediate mode UIs are based on text rasterization, ie polygon creation for each character of the text while retained mode text is usually featuring texture atlases containing all characters in upper and lower case. This of course introduces a limitation on the number of fonts available to the developer, font sizes, etc. My 3D engine is using an immediate mode UI for the editor and tools while it allows for creation of retained mode UI for in-game UI components by automatically generating texture atlases for the selected font types
