I think that "lacks endurance" is often a throwaway crutch as explanation for riders that don't excel in big classics as they're expected to. It's a bit like in tennis where every single player who gets hyped as a huge talent is later judged to be mentally weak. I think this is very frequently the case with jack of all trades riders that can be great stage hunters but lack the world class puncheur skills to be up there with the best in the world.