This has been my first year using Neem and I agree that the product is a bit hyped. It does work, but after using it for a few months I have noticed that it seems to build up on foliage and gives it a waxy appearance which cannot be good for the plant's respiration. Also, the bugs will just eventually move to the next branch or next plant until they find a place to munch that hasn't been sprayed yet. I have tried spraying the top layer of dirt on a few plants to see if it would help with the f-gnats with no effect, but I can't say what effect drenching would have.
I just received a small bottle of AzaMax which I used as both a spray and drench yesterday. I can say that I saw zero f-gnats this morning when I was in the greenhouse, I will check again this evening and try to get a better idea of the results.