Product review (3 stars): Don't stretch it out too much.
The strength of this duct is in the coils. While the coils are tight the duct is quite strong. When it's stretched out, it becomes ridiculously easy to crush. So, if you plan on using this in a space where it's length is not going to be compromised by too much stretching you're probably okay. This  the type of duct that is currently being recommended to prevent fire hazards. Just be aware that if you are putting this on an appliance that has to come away from the wall for installation and then go back in place, you are probably going to inadvertently stretch it past its stress limit.