I wouldn't like the visual noise that highlighting below-20% hp enemies would give.
As for the power level of KS: few enemies survive more than 5 hits. Bosses can receive +100% damage vs Discord and can apparently be frozen in a block of ice. I think with the current mechanics getting instant kills at 40% hp wouldn't be too far out of the scope of the reasonable. But KS killing enemies without extra actions being required would still feel nicer to me.
I understand why Dominate doesn't effect bosses, but it not being mentioned in the skill description made me a bit paranoid.
I think I misunderstood how the range of the freeze spell works, probably the kraken was out of range and I misinterpreted it as 'boss immunity'.