You get to see the inside of Cordelia's mind working in Shards of Honor and Barrayar; you never really do get to see through Dumbledore's eyes. Therefore, Cordelia does not look quite so scary and inhuman to we-the-readers; we've seen her indecisive, and we know how much self-possession it takes to pull it off, so we know that when it may look a lot like "I'm never wrong", it's also a lot of "guess and pray and look like I'm right so maybe I will be". We haven't seen Cordelia make a major wrong decision, like Dumbledore has (leaving Harry with the Dursleys, leaving Harry alone for Order of the Phoenix), but if she did judge wrongly, she would do it on a similarly grand scale.
If you don't know them, they're unstoppable forces of nature. Was it really Dumbledore who singlehandedly did all those things so now Voldemort fears him? Is he really that powerful a force on his own? Cordelia surely did not do most of the work beheading the Pretender -- Bothari swung the blade, it was Kou's swordstick, and Drou was the intelligence -- Cordelia was just the leader. In most of the telling, though, it was as if Cordelia did it all herself, singlehandedly. It was her breath that voiced the orders, and I guess on Barrayar, breath is as good as deed. Who are Dumbledore's unsung backers?