I don't think this is a bad feat for susanoo. If you pay attention to the itachi fight, it's never said that susanoo is actually invulnerable, only that it's shield is. If the body was invulnerable, then it would have no need for the yata's shield.
And given that the sword of totsuka evidently existed externally to Itachi before somehow being absorbed into the susanoo technique (???), it's possible the all powerful sword & shield that Itachi had aren't actually inherent to the technique.