Without customs, Marth loses (through my experiences, noticeably) to these top/high tiers:
Possibly also:
Out of the popular ("high tier") characters. So basically all of them bar
(which any could progress against us)
while also having hard/feasibly losing match ups throughout the game:
These may not be too bad/may get better, but either way, to have a bleak tournament presence such as this necessitates something less than high tier. This is coming from either playing these characters or playing against and studying with mains/great players of those characters.
In a lot of match ups, Ike actually comes out on top due to having a similar fair, but being longer range + more damage + better aerial mobility = safer on shield while also having a faster ground tool in jab for consistent damage, better throw game, arguably better recovery/kill capabilities. Marth doesn't really have good match ups right now because he's just way too unsafe on shield with
everything against
everyone.
This isn't trying to be pessimistic, so don't take it as such. I wouldn't put that many of these match ups (bar against high/top tiers) as super struggles. Even ness, pikachu and sonic can be played close enough a lot of the time (so soft-ish counters). But Marth has to be playing extremely passive in most of these as any attempt at pressure is frame trapping yourself essentially... stuff like fast start up long range disjointed tools will never be that bad, but misusing them is
death in Smash 4.