Well, if we're going to use the logic of amber belongs in the desert because amber takes millions of years to form and the desert could once have been a forest millions of years ago and the current forest probably wasn't a forest millions of years ago (which I disagree with half that logic), then the underground jungle still makes sense because of proper logic. Both of where amber is typically found and the fact most things from millions of years ago are now buried deep in the ground. Wait... Things millions of years old are buried deep in the ground.... Underground jungle is rather deep underground.... Hmmm....
In any case, I don't really like the thought of crafting it directly, I prefer the thought of harvesting it from trees. However, I also think 1-3 per tree is a bit much. I mean, I could make one tiny 100 block long jungle and harvest tons of amber, way more than any regular gem. Perhaps a 1 in 3 chance to get an amber from the tree? But... I also think there should be a rare chance for it to be an amber mosquito. Perhaps 1/100? 1/250? I won't dare say 1/5000 like from the extractinator, since that 1/5000 chance took me about 28000 tries to get one....