Over releasing could lead to problems depending on the horse you're riding. If you're a beginner on a push button lesson horse, and you're not doing any crazy turns after the jump or anything, over releasing is probably just fine. It can even be beneficial, in fact, if you haven't gotten your balance down yet. Better than popping the horse in the mouth over every jump.
However, I think with a lot of more advanced horses it can be dangerous to over release. Some horses might buck after a jump, and over releasing leaves you vulnerable for losing control if that happens—your reins are long and you're probably leaning further forward than you should be upon landing. Same thing if your horse is strong and tends to bolt or get quick after the jump.
Also, over releasing makes it hard to ride a line properly if you need to fit in a distance. I think it would be a lot easier to do a mediumish release over the first jump so you have the reins there when you need to tighten up the horse's strides to fit the distance. Also, of course, if you need to turn right away after the jump, having more contact on the reins can help you signal for the turn before you even land.
That said, I do admit to have a bad habit of over releasing. My lease horse is super sweet and doesn't act up much, and my thinking is that a big release, as long as I stay balanced, is like a reward for jumping. I want her to be able to jump comfortably, because I've seen horses that just get really annoyed with jumping when every time they jump they get popped in the mouth. But yeah, I'd say it depends on the horse, and the amount of control you want to maintain!