what happens if you hit a blind spot, or network congestion, and start buffering video while your drone keeps flying? I wouldn't fly FPV over 3g/4g networks, especially in urban environments. People can get hurt
The system can instantly adapt to available bandwidth and does not need buffering of frames like h.264 or similar codecs. So even with a very bad connection you will have a snappy video. If you loose connection completely - of course the video will stop. But the ground station software will continuously try to reconnect asap.
Signal jammers would be a problem as well.
They are a problem no matter what kind of technology you use.