Okay so first of all when I say top level I'm saying top 6/7, with qualifiers. Many people will say top 100 is top level or their local high PR'd player is top level but many of these players don't often practice or analyze or even play that much sometimes so that is not representative of a thoroughly prepared and understanding player to me.
I have noticed two very particular problems that get Marths blown up. There are others, but these two seem to be the worst:
1. Barely missing an opponent. Barely may mean in spacing terms, but often if you're drifting in this is a death sentence. Also, it usually leads to another problem, swinging a lot and getting into the heavy lag Marth has. So he pays a lot for being incorrect, or-
2. Inefficiencies kill him harder than other characters. If his punishes or kill setups are inefficient, he suffers way more than a Fox spamming usmash will eventually get lucky or a Sheik spamming Fair. As mentioned before, his lag hurts him a lot as well so if his movement is too excessive or he drops a combo he may end up close to an opponent in lag and off-balance which is very much the worst place to be for him. This happens a lot even in the case of someone like PPU, and it gets heavily exploited by Falcon and Sheik who can both prey on Marth's inefficiencies in various ways such as not getting edgeguarded well without a deeper flowchart.
I have noticed two very particular problems that get Marths blown up. There are others, but these two seem to be the worst:
1. Barely missing an opponent. Barely may mean in spacing terms, but often if you're drifting in this is a death sentence. Also, it usually leads to another problem, swinging a lot and getting into the heavy lag Marth has. So he pays a lot for being incorrect, or-
2. Inefficiencies kill him harder than other characters. If his punishes or kill setups are inefficient, he suffers way more than a Fox spamming usmash will eventually get lucky or a Sheik spamming Fair. As mentioned before, his lag hurts him a lot as well so if his movement is too excessive or he drops a combo he may end up close to an opponent in lag and off-balance which is very much the worst place to be for him. This happens a lot even in the case of someone like PPU, and it gets heavily exploited by Falcon and Sheik who can both prey on Marth's inefficiencies in various ways such as not getting edgeguarded well without a deeper flowchart.