I think that the biggest problem with area illusion spells is the need to sustain them. Which means investments on sustaining tools. Moreover, their drain is high. Or otherwise their radius is so small that if the enemy moves one step away, you loose the benefits.
So, you have to select between low force illusion spell or area combat spell. We can assume that both strategies have sustaining power (e.g. F5 focused concentration). I selected area combat spells. I can safely cast F7/F6 ball lightning/blast which almost kills/stuns everyone who stays on the area, they are difficult to evade (huge area) and no need to sustain. Drain is 4, but my drain pool is 17 (increase willpower sustained). Moreover, I can select combat spell specialization and mentor with combat spell bonus. And use also powerful clouts and stun bolts if needed. I think that the synergy is better than use of illusion spells and e.g. raven as a mentor spirit. In that case your stun bolt is weaker. Moreover, the willpower bonus is otherwise useful (higher full defense, better stun condition monitor, etc).
Yes, the best strategy is to have wide spell arsenal and use of many spell schools depending on the situation. But I still selected combat spell specialization and dragonslayer.