I'd pay more attention to the point where the asme had originally agreed to the ss keg design based on a safety factor of 10. That isn't exactly crazy for a pressurised vessel subject to sometimes rough handling and temperature variances in transit. The thermal expansion of the filled fluids is enough to expose any weaknesses. By designing the plastic kegs to barely 3-4 times serving pressure and not that much of a margin over handling points they ask for trouble. Of course, being a plastic it won't ever behave exactly to spec, manufacturing variability takes care of that. Once in a while some poor sod lets his guard down and gets caught out by a really close tolerance keg.
In fact, the euro kegs that come rubber coated are brilliant. They have lot if shock and rough handling tolerance built in. In the end, get what you pay for.