I see. I have seen people suggesting Roy's support list makes him decent, but personally, I am very skeptical of this, as less than half of his support options are particularly good units, and all of those are mounted units.
Re: SD Marth: I'd say what makes him bad is him getting screwed over by the game's changes more than anything else. In the original games, weapon weight actually mattered, the weapon triangle didn't exist yet, not everyone could get a class upgrade, and stat caps were only 20. SD made weight mostly a non-factor, added the weapon triangle (to Marth's detriment, as axes are virtually nonexistent after the early chapters), made promotion nigh universal (the classes that are exceptions get an extra 10 levels; this doesn't help his case), and increased stat caps. Also of note, the game added the Wing Spear, which is the Rapier on steroids. On top of all this, Marth doesn't even do great against the final boss, especially on anything above Normal, as then it's highly likely (or guaranteed, on H2 or higher) he's getting doubled.