Neem cake and oil

Hi everyone, this is my very first post, well, a question actually. I know nothing about gardening, and everything is learn as I go. I planted a lot of various young plants, and the soil I bought from Costco, which I used, I think, was infested with fungus gnats because it is everywhere now. I've tried everything and still have them little buggers all over the place. It doesn't help that we're getting a lot of rain here recently. We need it here in the Bay area but not helpful with the gnat situation. Most of the people online says to use neem cake or oil or both, which I've used both. I'm outside with my spray and yellow sticky traps everyday now declaring war. My question, (finally) is: what happens to my plants if I use too much neem cake and oil? I've looked high and low for this information but cannot find anything about it. Lots of info about how great neem is for getting rid of gnats.