Playing as lllians now myself; and yeah, the summons are kinda wonky.
Frostlings are 'permanent' on Tundra or Ice, but strangely, not on Glacial... which is weird. I spent a bunch of time spamming them for defense, and then suddenly they all disappeared. It's been alluded to, but I'm not sure under which conditions (if any) the Ice Elementals are permanent, as I never observed this behavior.
Personally, I think it'd be better if the Ice summons used the same mechanic as Death summons; one permanent Frostling per caster, who suffers terrain bonuses/penalties rather then a strange sort of conditional permanence; higher-tier summons can be one-turn only as normal.
As for the terrain, it's my opinion that the real strength of the Illians is that their Ice is basically awesome Grassland; awesome in the sense that even Ice hills provide 2 food. Everything turns into this Ice, and as such it provides enough food to let the Illians cottage everything (but resources and such, of course). If everything gets cottaged, then that's a lot of commerce winning the game for you. They're not the only civ that can do this kind of terraforming; but the fact that all the hills get cottages too, and that this doesn't cause food problems requiring strategic mitigation with farms, is awesome. Fringe benefit that you probably have lots of extra food from any food resources you have lying around; producing plenty of surplus population for whipping.