The satellites are expected to control their deorbit, just as they controlled their ascent, and neither controlled ascent nor controlled descent pose any threat to ISS. If a satellite failed in orbit and was unable to control its deorbit, it is still very unlikely that it would come anywhere close to colliding with ISS on the way down. And in the very unlikely event that it did, again there would be plenty of advance notice for ISS which could do a maneuver to avoid it.