
Fine , autokill compared to practically every other attack in the game (the only other that comes close is possible a giants grab attack) And they do auto-kill war machines. But you get one, thats one, chance to pass a test after which you are dead regardless of how many wounds you had, whatever protection you paid for , the cost of the model etc , even cannonballs are't quite so effective.
The type of test doesn't always scale that fairly either , most of the Tomb Kings army for example can be considered fodder to these types of spells, especially the big expensive ones. Dwarves probably just get by on thier dispelling ability (which probably will be toned down in the next book)
I would venture that GW have already realised that "auto-kill" (please note parenthesis) spells do not do much for the betterment of the game, if the latest Spell lores in the books are any indication. My
personal favourite spell is the rune maw spell.