I don't think it is dangerous but there are far better options. Instead of purchasing expensive cricket feed you can do better by just looking in your kitchen.
The crickets will do fine on it and the mantises fine eating them, but like Rick mentioned there are better cheaper options. Mantises don't need tons of calcium and the other stuff that is placed in gut loads designed for reptiles and such.
Flukers will be fine. After all, that's what it's made for. I've used it for years to gutload, also adding oatmeal and fish food flakes. Also just discovered today they are WILD for collard greens.
I've noticed that crickets fed only with fluker's diet die much sooner and tend to be thinner than those supplemented with fruits and vegetables and those fed only with fresh food.