That's because he promotes late. It's not that he's actually weak.
Only because IS thought it would be a great idea to promote him near the very end of the game. So the Binding Blade was horrendously broken to compensate.
Binding Blade should at least get healing though.
Roy is a late promotion but he's a good unit at the core, but then he gets one of the most broken weapons ever in the series even considering the family-heirlooms from Hoshido/Nohr.
Roy could promote early and he'd still be incredibly mediocre. His main problem in FE6 is that his base stats suck ass and his class is underwhelming.