... they're extremely vulnerable for their unit type:
Only toughness 4.
Only a 5+ armour save which will be negated by most things opponents will launch their way.
Only WS 4.
No ward save or regeneration (except for the Necrotect bubble).
No monstrous infantry type characters that could be added to them.
Six of these guys will set you back 300 pts. .... and investment which they're not worth off.
In fairness, finding Monstrous Infantry that aren't Trolls with better than WS4 T4 5+ armour is unlikely (Ogres can get 5+ armour with a 6+ parry, but are WS3 which is a big defensive disadvantage). However, said Ogres are 32pts with an Ironfist, which makes them a hell of a lot more viable since you can get 3 with a few points change for the price of 2 Ushabti. That you can horde up Ogres is a huge plus for them - I think Monstrous Infantry aren't viable unless they're cheap (so can be brought in big units, like Ogres) or are very resilient (River Trolls being -1 to Hit and 4+ Regen, for example), and preferable with good protective buffs (Trollguts/Toothcracker, Lore of Nurgle). Ushabti are probably pretty good with Great Weapons in a unit of 8 (two ranks of 4), but they're prohibitively expensive at that point and their only defense is relying on the Incantation of Protection. They'd be a solid unit, I feel, just very pricey for it and no better than the equivalent points' worth of Necropolis Knights.