marths smashes are definitely stupid slow. his fsmash is seriously tooooo slooowww. i can't powershield -> fsmash anything with half decent speed.
i think marth's biggest weakness, which is not necessarily a weakness, is simply that he has a steep learning curve. a good marth, even a decent marth, requires a lot of practice and perfection in spacing, reflex, fast finger training, precision of attacks, and careful recovery among mastering his best moves.
you might say that becoming a great player with any character requires the same dedication, but i would argue marth requires a fair bit more than most. it really doesn't take much to projectile spam and camp, but getting around said spam and camp is harder, which is what marth needs to do. because of marth's piss weak non-tipper smashes, his spacing issue is much great than most characters since as far as i know, DDD's dsmash will blast you the same way whether he hits you with his hand or the head of his hammer. further, marth is fast, but not so fast that you can do well with even ok reflexes. due to his lack of a spammable kill move and comparatively low reach now he must actively look for openings and take them at a dime, all the while avoiding his opponents kill-move-spammage (cough MK cough). marth is slow enough and light enough that he can't afford to make mistakes, where some characters can get away with them due to raw speed, or survive their mistakes due to weight. as such to play a good marth requires a perfectionist level that many characters do not depend on as much. I mean broken *** MK can mash buttons and still create combos and he great grabs, dash, and insane speed AND reach. for a marth to beat some of the higher tier characters, a significantly higher level of skill and devotion is required. well, of course you need to be better than someone to win, but what im saying is that the marth player needs to be a better by a larger margin.
its simply because marth really takes a lot of work and learning, or naturally raw talent, that makes him very anti-noob. while a lot of characters are rather noob friendly.