To me they seem good on paper, but really are not great. They have no armour, They still can die to combat res, they have a single attack and they are 30 points each. For 6 points less, you get black knights with barding. That and they need to be within 12" of your general to march.
I have a feeling black knights will be pretty terrible in this sort of build.
Another thing, I really don't think you need two corpse carts, both with unholy loadstone. I would take a single Balefire. Repitition is good, but they are move 4. Not the fastest things in the world. And you can only move one a turn with Kemmlier.
In this build loadstone is essential turns 1 and 2. The more I can avoid getting them all cannoned turn 1 the better.
If your trying to make your BSB harder to kill, I would drop the magic weapon and take a dawnstone instead. That way your getting your armour save of 3+ with a reroll, and then a 6+ ward save from HW&S. Or give him the seed of rebirth for the mortis engine to buff to a 5+ regen. And if your hit with fire you get a 2+ ward from the helm anyway.
Don't care about him dying that much (he's a wight, he doesn't do much dying), he's there to punch characters in the head so that Kemmler doesn't have to bite it. He does that in this build. The BSB is mostly an afterthought with spare points, seemed ok.
I feel the zombies are a little small unit size... 30 would be good.
In this build 30 doesn't do anything that 20 didn't already do. Remember that I'm either casting Invocation turn 1 and 2, hopefully with lodestone or I'm probably losing the game. So I'll be at 45ish for each unit end of turn 2.