It's not harmful, but it can give you advantage, for like distraction (everybody is focused on the desynced player while his teammates can easily take them out).
Yes, it happens too often. His ping always looks fine and it always happens after I've killed him a few times. First comes the reconnect spam, then he does that desync shit. In the video you can clearly see his ping is perfectly fine, but if that video is not enough, I'll be capturing more of his gameplay.