While I wouldn't want to build an AR based around a plastic lower, I really don't think there is any validity concerning failures and breakage. I have a Carbon 15 pistol, and shoot the heck out of it - the plastic receivers don't show any sighs of fatigue or failure.
These things fire 5.56 in semi-auto. Contrary to the anti-gun media, the AR is neither a high-power nor rapid fire weapon. Maybe, just maybe if someone used one with an RDIAS doing betamag dumps, the plastic receiver might eventually fail due to high heat and stress.
The real reason why most people object to plastic receivers is because they have no class - not nearly as pretty as a 7075 Billet Receiver set.