Question wrote:In the end you are spending 200+ pts on a large unit that has a 100% chance of failing a strength or init save or die...
While this is true, you have to remember the range on those spells is 24 inches. The range on the cauldron is 24 inches. However, the cauldron goes at the beginning of your turn. It is very easy to place the cauldron far behind your lines, where the only way for the enemy mage to get to it is to move up at you, which is good, because you generally want to be in combat with a LVL4.
Sure, there are times when he can safely get up and pit/dwellers the cauldron, but it's a lot harder than you might imagine. You might lose out on the
BSB some of the time, but you have to just balance the decisions.
Moreover, you need to consider the flexibility. 7 to 10 attacks on your horde formation, when you have hatred (and likely mindrazor or withering) is HUGE. It's also huge to buff your cold one knights and add 5 to 10
str 6 attacks on the charge. If you have a deathstar, you can also protect it on the way in. You need to think of it a little differently. There's likely 1 influential combat each turn after the first one or two turns. If you can buff each of these combats with +1 attack, you've effectively said "my cold one knights have +1 attack, so do my spears and so do my blackguard" and "my blackguard also have a 5+ ward against shooting"
Let's not forget about killing blow allowing you to deal with a lot of armored characters or knights.
For 200 points that's a great buy. Being able to put the
bsb on there for 25 points is just an added bonus.