Well all the regulars here know the spiel by now, but I'LL HAVE MY RANT STILL.
Well, for starters,
he has a slower walk than like, a third of the cast in most relevant situations due to the non-existent acceleration.
This means that he may be walking faster than Fox, but this is after a second++ of him "slowly walking up to you", but without the PogChamp. Up his walking acceleration and this would truly be relevant (hmmm tasty walking jab, it would almost be as good as sheik's walking ftilt or fox' walking reverse up tilt).
Furthermore, I don't think it's his neutral that gets hated on that much, he has some of the best footsie tools in the cast.
His paramount issue all this time that gets highlighted in almost any match you watch a good Marth play against a top tier in tournament is his feeble reward for getting successful hits.
This means that many characters can play a harsh attrition game against him, and that's where his neutral (these terms are horribly overlapping, btw) starts to fall apart because in turn of him taking more risks, he's still getting little reward. And let me tell you now, this character is FULL of risks.
Marth still has no grab reward or real kill set ups. He had one semi-relevant set up in shieldbreaker deleted. He has extreme reward in something like his forward smash, but that's truly nothing more than the sexiest whiff punish in the game that comes with 40 frames of lag.
Thankfully they've made him usable competitively by having three strong (when spaced well still) aerial kill moves, but that's almost all he has that's "good" - raw KOing aerials. But I must highlight that even though they're some of his "best moves", they're still not some of the "best" moves in the cast, above average definitely but I wouldn't put them much further than that.
Marth could be a lot better in every way with his mobility specs buffed, even considering walking (he got a lot more out of in Brawl) he had all of his mobility specs comparatively nerfed between games.