Polyethylene is used in many applications. As Dyneema and Spectra, it is used in bulletproof vests. Manufacturers have also used polyethylene to make rifle protection plates, as an alternative to steel and ceramic. So it has a proven track record in protection. The French Army currently uses a helmet made with Dyneema.
However, polyethylene is very sensitive to temperature. BulletProofMe.com, for example, advises against exposure to temperatures less than 15 degree Fahrenheit nor more than 175 degrees Fahrenheit. Ultraviolet exposure will also degrade its performance.
So it remains to be seen if the vendors have fixed the temperature problem. The US Military definitely will have to replace their helmets much more often to avoid performance degradation.