So.... The price of a demigryph knight... Compared to ushabti... We get the shaft! With the (free) halberd upgrade, they throw off S5 attacks like it's going out of style, have 1+ armor, M7, and I3/I4. For 8 points more. WS... I think that's at least as good as ushabti.
Maybe they're better compared to snake surfers, but I could never bring myself to like those, so I don't know the stats.