Recently I've been programming with UDK mobile and have come across a few limitations that I couldn't find a whole lot of documentation for.
One of the limitations is that terrain isn't supported as of yet in UDK Mobile. If you attempt to start UDK Mobile previewer using terrain it will crash the player.
Another limitation is that Dominant Directional Lights aren't supported either which is sort of a pain because this is a nice way of creating sunlight type effects. This can easily be worked around though using a directional light. You can use a skylight to brighten the level up a bit as well.