Mealworms are super cheap and super easy to care for. Throw them in a bin, feed once in a while, and refrigerate for longer storage. They are low in fat and high in protein, but they have a nasty rumor that follows them around.
They cause impaction.
For years people fed their dragons mealworms as a staple without problem, and those who ended up with problems almost undoubtedly had something off in their care. Reptiles are made to digest crazy things from bugs to raw meat (even though for beardies that isn't recommended). When impaction happens, there is usually an underlying issue. Dehydration and lack of proper temperature are both major causes. Water is needed to aid in digestion, and heat is required for digestion. Paired together, a dragon will be able to eat, but not digest, leaving the food to get crammed up in the digestive tract.
Mealworms are pretty decent nutritionally speaking but easily beat by superworms, silk worms, Phoenix worms, and dubia. So yes, mealworms are able to be fed. Yes, they are safe. No, they won't impact and kill a healthy dragon. Just make sure your care is exemplary before feeding to minimize risk.
Feeding mealworms, no matter the size, to a dragon under 12 inches is highly advised against. For adults they make okay treats, but for babies and juveniles, they can cause a health risk that can end in the death of a dragon.
***By no means am I saying go out and use mealworms as a staple feeder, I'm purely trying to get rid of the 'automatic impaction upon eating a single mealworm' myth.
"Proper husbandry (lighting/temps/hydration) is extremely important to keeping a healthy bearded dragon!"