Missiles can be shot down by state actors, non-state actors can hide deep in areas which are out of range of the U.S. or its allies. But launching anything on an ICBM makes it almost impossible for other nations to distinguish from a nuclear missile launch.

Of course, if Prompt Global Strike is capable of carrying cargo with it then maybe countries like Russia or China would suspect it can also carry a small nuclear warhead, so it may be that concern doesn't completely go away.