A few random thoughts:
1) BF Ulm *is* powerful. But to me, it's more because of the cheap (and stealthy!) crossbow, free wolves and other good units. VCs are more like icing on the cake. But on the other hand, I'm not very good with Blood. My estimation could be off.
But still, all of these advantages are from regular troops which are more useful in early game. With the weak mage of BF Ulm, the late game is hard to say. Balance in late game is very hard to test with solo as AIs don't have enough varieties of tactics.
2) In most MP games I played, most indies were taken before turn 20. So, I don't know how well IKerensky's expansion speed will manage in such situation.
3) BF Ulm doesn't have cheap blood hunters. The Second Tier has one Blood and cost 190 gold. It's reasonable but far from cheap. You might get one Blood with Fortune-Teller but the chance is 1/4. But for course, I agree that Fortune-Tellers are good units even though they're vulnerable to "Mind-Duel"
4) VCs are not really very good platform for super-combatants. They have too few hitpoints and immortality doesn't help because you want to fight under enemy dominions. Moreover, even when he's immortal, any temporary kill will cost you the items on him (if you didn't win the battle). It's not a small loss.
More importantly, you want your super-combatant to actually *kill* your enemies, rather than routing them. A VC doesn't kill fast enough with Life Drain. You can make him reasonably powerful using the "Snake Poison Bladder". Doable for BF Ulm and not a bad idea at all (just Construction 4). "Soul Vortex" is great but it's Alteration 6.
But still, a VC should be easily overwhelmed by unroutable undeads, vine creatures or earth magic constructs. All of these are lifeless - do anybody know how much they're affected by the VC's lifedrain?
Moreover, if your enemies have "Flaming Arrows", "Flaming Weapons" from blessing, body ethereal won't help. All Ice weapons of Caelum should (still) be considered magical. Your VC will be sitting duck versus a few Caelum infantries, provided that they don't rout
. Etherealness doesn't help versus most spells neither, one "Blade Wind", a few "Orb Lightning", "Solar Ray" or a single "Fireball" could easily wipe out your puny 25 hitpoints.
These are all the basic vulnerablity in additional to those specially for the undeads... A single VC is reasonable powerful but it won't be a juggernaut versus any non-indies. Ten might be different - but well, with 440 blood slaves, you could have 17 "Horde from Hell", 8 Ice Devils. All are mighty armies.