I know some points made indicate against her, but Siuan seems like a very good fit to me. Sure, in NS she wanted to get strong enough to ask Cetalia to leave her alone, but she clearly started loving her job. Then they made her head of the Blue's Eyes and Ears, a highly influential position in the Ajah. She also got some sort of visible administrative role which Seiane was able to use to convince the Hall of her ability to be Amyrlin, and this can't have been a very small thing.

Then she got picked to be the youngest Amyrlin ever, just 9-10 years after gaining the shawl. By this point she must have been very close to her final potential, and already equal to the likes of Moiraine and Elaida. But maybe she was just a smidgeon stronger? Not enough that anyone would remark upon it, but enough to make it to the notes. She just never had the incentive to hold the OP for long stretches of time, or tax her abilities like Moiraine had to. She'd be a perfect candidate for not reaching her potential, yet being in a position that no one would dare point it out.