Powerfull? Perhaps in right hands, but
1) invokers need top time for preparations.
2) Anyone who can cast/commune 'word of recall', stun/choke/spinebraker will survive most of the time.
3) You will have to spend a lot of time spamming area spells, and figure out how to master direct spells
4) aura/shield always, barrier for tough matchups.
Evil invoker:
1) Marans have permafly from mantle, so if you wish to kill em you need earthbound with scroll/wand and curse with scroll/wand somehow inside quicksand.
2) Marans are immune fire (no immolation, no nova, no fireballs)
3) Monks have direct spell avasion that works about 1/3
4) from five paladins i have fought two had "light of heavens" dispeling virtue
One dwarf-paladin-monk i have fought had undispelable earthshield (imm reward or tattoo)
lets count who is broken:
1) permafly
2) immune to fire
3) immune to earth
4) resist magic (resist acid/cold/lightning immune to the rest see above)
5) direct spell avasion
6) dispelling virtue
my two round cone of colds with 7 affinity were doing about "evisicrate/dismember" to him, he could do 10-15 wound/mauls with punches during it throgh 5 shields + ABS.