Tbh, I am not so sure on that assessment, it depend on how you build around it. Looking at taking part in a series of games at my store, Kroak + templeguard + Astrolith also using a balewind vortex, and you get a pretty capable army nuker. Especially with the 4 spells of Kroak ensuring he can either sit on top of his balewind with +1 save and +3" are denial, keeping him out of close combat, and potentially reach the 2+ save combined with his rules. I guess it would truthfully end up just how much damage that he will end up doing, what with a +2 casting modifier, a *2 range on his spells and +8 range. Add in a Priestly trappings Skink Priest and you end up with rerolls happening out the wazza.
The key is, you would seriously need to build around Kroak to get the best value out of him, but don't we already do that?