Don't forget that another part of his power was his huge ass army and the way he manipulated people, he had power in more ways then just using darkness. Regarding what you said, I agree that it would be sloppy writing but there is only much youn can do from a first person perspective. We never saw the Darkling in a real fight so we never had a chance to see his full potential. We also never see him fight Alina so calling her stronger is wrong. The only time it got close to them fighting was in the last book but the Darkling was already weak from using magic.

In order to make him seem like a better villain there should be a solo book about him so we can better understand his powers.