Terp1answered grow question 3y ago Neem prevents chitin production during the growth stage in insects. This means it won't do much against fully grown insects. Neem helps to stop insects from reproducing in or on your plants.
It would definitely help with spider mites and fungus gnats. But please don't use it during the flowering stage, as neem is also not completely safe for humans. However, neem is no problem during the veg phase.
In my opinion, the most efficient method is a mixture of neem oil, potassium soap, and water. You can spray the leaves with it and/or water the whole plant.
Happy Gardening 💪😎