catfood is used alot in home made gutloads because it contains alot of protein and other basic ingredients good for crickets and other insects. But I wouldn't feed catfood or dogfood to a bearded dragon directly because it is far from nutritionally complete for bearded dragons. It also has stuff that wouldn't be good for your dragons, especially adults. There is typically too much fat and protein in dog and cat food than an adult dragon needs and not enough of what he does need like calcium and vitamine D3.
To my knowledge insects feed on a diet consisting of cat/dog food as well as other ingredients does't have any harmful effects on dragons and other reptiles that eat the insects. All creatures break down and reform what they eat according to what their body specifically needs. Even chewing alters the food eaten plus digestive ensymes added in the process. Required raw nutrients are then extracted via the bowel into the blood and recreated/utilized in chemical reactions to do what is needed by the body. The end resulting proteins in crickets is probably alot different than proteins that came from catfood. Yes I am guessing on this part but to me it does make sense..and no I am not going to spend a few hours online trying to prove my theory is right or even wrong, LOL.