I have used Video chat app using QB sample credentials. I have 1 video chat app installed on both of my iOS devices. When both devices are on the same network the video quality is much better but when both devices are on different WIFI networks the quality becomes poor; the video becomes very jerky and the audio sound becomes unclear. But at the same time Skype on both devices the quality is very fine and clear.
It is the peer to peer connection; it only works if both users are in the same network. If 2 users are in different networks - peer to peer is not possible and all traffic goes through relay server.