Just like with dog food, dog treats vary wildly in regard to quality. And you'll want to keep a keen eye on the very same ingredients that one must be wary of with kibble.
Steer clear of:
Chemical flavoring and coloring can cause terrible allergic reactions including skin issues (rash, itching, and/or hot spots), coat issues (brittle, dry hair and/or thinning hair), and/or gastrointestinal distress (general upset stomach that can interfere with a healthy appetite, vomiting, and/or runny stools).
Synthetic dog food preservatives can be very detrimental. These can cause all of the above reactions, and can be toxic to pets, including ethoxyquin which is under investigation for causing liver and blood issues.
By-products and generic meats.
By-products are parts of animals that are deemed unfit for human consumption (beaks, undeveloped eggs, lips, etc.) and generic meats are even worse (can legally include animals that were dead or diseased en route to facilities, road kill, and even dogs and cats that were euthanized).