marth's vert is fine, he has his double jump, db boost, and ds. more accurately would be his lack of horizontal boosting while recovering which makes recovering from an attack with a ton of horizontal launch very difficult and also forces marth to recover low or high in most situations.
@Kishin, marth's biggest weakness IMO is his insane learning curve. while he certaintly possess the tools and means to compete, you must be flawless to put those tools to use. many of marth's advantages and tricks require your timing to be extremely precise and your smallest mistakes can cost you greatly. other high+ tier characters don't have this particular weakness because they have things like laser projectiles, super speed, broken tilts, kill you at 90% attacks etc.
also, if you break their shield, attacking them in the air is pointless. just let them fall and charge another SB or a smash.