Darnic being on the level of Kayneth still kind of implies that he would dominate in a competition with most magi. Especially since part of his danger is his longevity leading to experience that could tip the scales in his favor even against talented magi.
Remember that for all intents and purposes Kayneth was the superior "magus" compared to anyone else in the 4th HGW, with Kirei being an aberration and Kiritsugu being able to hit way above his weight-class due to tactics+origin rounds. So Darnic only being a Brand isn't necessarily a mark against him.