Chitin lacks any real significant nutritional value. It's a structural molecule and as such is very low in nutritional value. It's similar to keratin you find in mammalian nails or hair. Not very nutritional if one was to eat those. Overall there isn't anything overtly negative about chitin. It's just a structural compound (it's an arthropods skeletal system). That's why when we as hobbyist are feeding our amphibians or reptiles it is important to not give them to large of insects to eat. It would be equivalent to a human eating only the skin of a fruit. This is mostly fiber, which many types are insoluble and thus non-digestible by mammalian enzymes. That's why when you look at a Herp's fecal matter you can see the exoskeleton of the cricket, roach etc of what it ate. This is also why insects with more chitinous exoskeletons have an unfavorable inverse Ca:P ratio. Phosphorus is extremely important in structural molecules and more will be present with an older insect (because the body is producing more P to create this durable exoskeleton). The only organisms that can readily digest chitin is some species of fungi and bacteria (such as those found in ruminants - such as cows) who can produce enzymes to break down chitin and lignin (another structural polysaccharide). Hope that explains it haha.