I'm pretty sure people call him the worst Merc because of the backtracking/grinding needed to make him usable. Even Jesse (garbage) can barely reach Myrmidon before Chapter 4 with some love, but not Atlas. Also, high Skill is actually good, because the Brave Sword is pretty much the best thing; it makes Saber and Kamui's lower Str less meaningful.
Also I think my Gray got screwed lol.