Personally: My dogs have been serious, serious chewers and Nylabones presented no digestive problems or risky effects whatever. In their cases, Nylabones were far safer than the alternatives -- because they could just gnaw at Nylabones, whereas they ripped hunks from other chew toys, creating a choking or digestive risk.
-- Nylabones come in different "strengths." If your dog is a mighty chewer, choose durable Nylabones that are meant for powerful chewers.
-- Skip Nylabones if your dog is so powerful that he-she can tear off chunks of the thing instead of just gnawing away. That's a choking plus gastro hazard.
Again, this is my personal experience, Dogs have to chew *something*, and -- for me-us -- Nylabones had more advantages and fewer risks than rawhide, breakable natural bones or wreckable chew toys.