It's not that it's hard to do, but rather that you have to remember to do it in the first place and that you might be doing things elsewhere that make you time it wrong or forget to finish the job. This is the same sort of thing that happens when you send units to a planet and then accidentally forget about them for a while. Anytime this sort of thing can be dealt with by the game in a reasonable way, it really should be. Right now, mixed fleets that contain teleporting units are frustrating things to work with.
