Neem oil is a great ipm to use before problems arise and while in veg. But most people with an insect issue are looking to get rid of the issue as fast and as safely as possible. Neem doesn't kill gnats or insects. It breaks their life cycle, preventing them from feeding, reproducing, ect. That means you have to use it for the entire life cycle of the particular larva or insect. It could literally take weeks to work. Diatomaceous earth creates kill zones. It's more immediately effective. They both work, and both have benefits for the soil. It's all personal preference. Whatever works best for each grower and their situation.