I haven't played the boardgame, but for the iOS version on Legendary difficulty, I'm not a fan of Illusionist. Any monster you Mind Trick with Invisibility is a monster you haven't cleared, and it can end up getting shuffled back into the encounter deck the next time you cast Augury. I find the +1 hand size of the Evoker more useful, even if the other abilities are blah, along with packing a couple Fire/Lighting Bolts for anything Swipe can't handle. My ideal loadout for him would probably be...
2x Swipe
2x Firebolt
2x Haste (Haste is amazing on Legendary, extra explores plus movement to help deal with the adjacent movement requirement)
2x Incendiary Cloud
1x Scry
2x Augury (Replaced Scry once later story mode chapters allow more points in Recovery)
That said, I agree that he is the best support, especially for larger parties on Legendary. His titanic hand size means that he'll almost always be holding a decent set of location agnostic assist tools, and not being dependent on blessings means that he can spend them at will without worrying about draining his deck. The one notable downside is that he functions mostly as combat assist since the skill buff spells are dramatically inferior to blessings, so much so that not even the ability to recharge them makes up for it. Not a huge deal, though, as combat rolls are the most common in the game so he won't spend any time gathering dust.
Seoni, for her part, is no slouch. She's probably the second best support on Legendary (Lem is kinda trash) and is a much faster explore than Ezren. With 2-4x Haste and 2x Scry she can tear through location decks with abandon.