Hardness is the first safety filter
A widely used rule of thumb among vets is the "knee test": if you wouldn't want a chew hitting your kneecap at full force, it's likely too hard for your dog's teeth. Very hard chews, some antlers, hooves, and certain synthetic bones, are a common cause of slab fractures, where a large piece of the tooth's enamel shears off under repeated pressure from aggressive chewers. Softer, more flexible chews that give slightly under pressure are generally the safer long-term choice, even if they don't last quite as long.
Size relative to your dog matters as much as material
A chew that's appropriately sized for your dog's mouth reduces the risk of it being swallowed whole or getting lodged across the back of the throat. As a general guide, the chew should be noticeably larger than what your dog could fit entirely in their mouth, so continued gnawing wears it down gradually rather than allowing a bite-sized piece to break off early. This matters even more as a chew gets used down over days or weeks, a chew that started out an appropriate size can become a choking hazard once it's worn down to a small nub, which is a common point where owners get caught out.
Why "unsupervised" is doing a lot of work in that question
Most vets are cautious about recommending any chew as fully safe for extended unsupervised use, because the risk profile changes with how your individual dog chews, some dogs gnaw patiently for an hour, while others crack pieces off and swallow within seconds. Even a chew that's generally considered safe is worth supervising for the first several sessions so you can see how your dog handles it before leaving them alone with it. Once a chew is worn down to a size your dog could swallow, it should be taken away regardless of how "safe" the original product claimed to be.
Signs a chew has become a risk
Watch for a chew that's cracked, splintering, or worn to a small enough piece to fit fully in your dog's mouth, all of these are cues to remove and replace it. Gulping, gagging, or pawing at the mouth during or after chewing are signs worth taking seriously and, if persistent, worth a vet visit to rule out an obstruction or dental injury. Digestive upset, vomiting, or reduced appetite in the day or two after a new chew can also indicate it didn't agree with your dog.
When to check with your vet first
Dogs with a history of pancreatitis, dental disease, or previous choking incidents may need a more conservative chew selection than a general product recommendation can offer, and it's worth discussing chew type and hardness directly with your vet for these dogs. The same goes for puppies still losing baby teeth, where very hard chews can cause unnecessary discomfort or dental trauma during an already sensitive stage.