Join Date: Mar 2011
Location: Heavily Spanking Saint Celestine
Sorry for the late reply, but the WHFB section on these (and certain other) boards seem to be rather dead in the era of AoS. A pity, since both games could co-exist quite easily, since they appeal to very different kinds of players.
As for your questions about the Ushabti:
Since they have the 'Arroes of Asaph' rule, they always hit their intended target on a 5+, ignoring all modifiers, which can be quite good when you're facing a lot of negative penalties. Thus their BS of 2 is not that big of a deal.
Personally, I'd never run them with Great Bows though, 'cause like you noticed yourself, 50 pts. for a single STR 6 shot which isn't even remotely comparable to a bolt thrower is extremely expensive. Better invest in mass shooting from Skeleton Archers if you want to customise a Tomb Kings list for shooting (possible, but has its drawbacks).
Giving them additional Hand Weapons (for +1 attack) seems redundant as well, leaving you with a model with 4 attacks at STR 4. You don't need to field monstrous infantry to dish out STR 4 attacks to be honest.
Great Weapons seem the way to go, since with their initiative score of 3 every enemy and their mothers will strike way before them anyways, leaving them to go last more often than not, so adding ASL to them doesn't change much.
So, 3 STR 6 attacks seems not so shabby for monstrous infantry, but ...
... 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.
IF you deliberately want to field them due to fluff reasons or army visuals, then go for the Great Weapons, but don't expect miracles from them.
40k Armies: Dark Eldar / Adepta Sororitas
WHFB Armies: Vampire Counts / Empire / various Elves
Horus Heresy: Alpha Legion / Warp Cult