I think Dark Willow's plan to end the world was probably a bit overly ambitious, especially considering the fact that her power didn't seem to last that long before burning out. First time she burned out, she drained Rack. The second time, she drained Giles' power, which was only on loan from the mysterious "Coven" back in England, and that was a trap, meant to have a huge initial flash, giving her the illusion of tremendous power that actually burned out quickly when she actually put it to the test. Raising the temple exhausted it almost completely, leaving her with not even enough power to sustain an extended attack on Xander. When all is said and done, the girl is a pure flash in the pan.
Illyria, on the other hand, was an awesome fighter, even after a portion of her power was drained, harnessed, diverted, or whatever Wes really did with it. She was at least as good a fighter as Angel, and she had probably thousands of years of fighting experience before being entombed in the Deeper Well. Willow has had, what, seven years of ASSOCIATING with real warriors. She was good at taking charge on the couple of times Buffy was out of the picture (at the end of Season 5 and the beginning of Season 6) but even then, she never did much real fighting, just standing back and giving orders.
The one time that she fought Buffy, she drained her power almost to depletion, and we're talking about a Slayer, not a world conquering hell god. Willow is nothing more than a splattered bug on Illyria's windshield.