BSFL consume untreated organic waste which is a great sustainability opportunity. FLYGRUBS only consume traceable human food waste... all the food you throw away after dinner is what we raise our larvae on.
Mealworms can not directly consume a lot of water and they basically only eat grain byproducts (which in many countries humans actually eat, such as wheat bran and the like). This means they compete with humans for the same resources. It also means they indirectly consume a large sum of water since these grain byproducts had a large share of the irrigation water.
Black Soldier Fly larvae is a more environmentally friendly feed additive than mealworms, for your backyard chickens, wild birds and ducks.