These work very well! They are plenty strong and quite flame resistant of course. One suggestion if you’re using them in a fiberglass body tube, sand the edges of the tube and wrap a little bit of duct tape around the cord to avoid abrasion.
These are very durable, and although not totally fire proof, you don't have to worry about them getting damaged by the ejection charge like the elastic shock cords. Just make sure they are long enough (10 - 12 ft) to avoid damage to the rocket.
Strong, light and you don’t have to worry it burning up. Why use elastic or nylon?