More of an observation than anything else.

Critical effects only trigger on (and effect) the selected target even with an AoE.

Something of this sort is essential otherwise, at higher levels of attack bonus, Evoker's will be setting entire groups burning with every area spell and Clerics with Auspicious Critical will just power up by whirlwind-ing goblins.

This "primary target triggers" thing results in some interesting tactics. For example in the example above the Evoker should select the toughest target as the focus of their AoE (to get best use of the stacks) whereas the cleric with Auspicious Critical is going to want to select the weakest target (to optimize the crit chance).

Is there is an alternative way to do this in the game mechanics? I am not sure.