Well, there are mechanical problems, since the tube has to expand and contract and deform, so would the coating. And anything that behaves like that is going to have properties that are similar to rubber. Various rubbers have different diffusion properties; as was noted up the thread, latex tubes lose air much faster than butyl ones do. In fact, may latex tubes get a thin coating of butyl rubber on their outside surface as one of the steps of manufacture. (That's the inside of the finished product, inner tubes are made inside out.)

Some butyl tubes do noticeably better than others. I've found that Michelin Air Stop tubes are slower leaking than most others, but even within brands there can be substantial variation. My commuter currently has two nominally identical generic tubes in it. I check the air pressure once or twice a week, the front has always gone down from 50 to 40 or 30, the rear is always much closer to the 60 I inflate it to. (only the rear tube has been patched...)