He just has a hard time making his strong points work, or they don't mesh well together. He doesn't have any decent ways to combo into a Smash besides on FF's, his usual killing aerials send people more diagonally up than he'd probably like, the attacks that linger tend to linger too much, etc. His positives all tend to have an accompanying downside that's not good. Oh this kills good? Startup. This move lingers? Won't be very safe on shield. XYZ move gets them away from you? Good, but now you can't combo them.
It's frustrating playing him when you KNOW exactly where xyz Mario (or "easy" combo character) combo starts, ends, what random hits you can pull it off from, the variations you can add in like different moves/how platforms affect it/changes based on DI or character... etc and very few of those apply to G^W. Besides the obvious vertical setups, there aren't "I tipped with Fair so I get a free Dair spike always" during the middle of combo's moments.
There are the usual and understood "Here's my gimping here's my best answer against most approaches" but there's a hard time naturally flowing and linking his attacks together in meaningful ways when compared to a lot of characters in the cast. You tend to maybe get a launch or two, get a Nair or Fair, and you're done. On FF's alone, more vertical juggling which isn't bad. Except on FF's where this transition might set up for a quick edge guard, that's not a strong punish compared to what a lot of the cast can do, and is almost like resetting the situation with a small edge for G^W, while he can still die quickly and from 1 touch because his opponent's character is geared towards it better. G^W combo does 30% (maybe bit low, say 40%), Falcon combo or tech chase or xyz will usually do more with less effort. Etc
On the strictly negative side, being light weight with a bad horizontal recovery AND being sorta bad at getting out of combos (besides Upb, which is OK) doesn't pair well. Good vertical recovery but using it at lower heights also gives people a ton of time to react to it. Poor grab range makes it harder to take advantage of his pretty decent throw game, and on a more minor note the moves you're likely to land on opponents barring CC dtilts don't set up into grabs very well. His shield pressure is overall poor: his ability to poke at his longer reach IS good but his ability to get up close and threaten you or close the gap with a multi hit move/solid frame data move doesn't exist. He's not Shiek Fair or Nairing your shield, or doing aerial to jab mixups that work for other characters. His overall startup on moves has got to be higher than the cast average, and his defensive options are not cutting it. Spotdodge Rolls and Tech roll frame data/distance are lacking, which sucks for a character that cannot afford to eat punishment.
He can be really fun to play, especially vs Fast Fallers because that's the 1 main time you FEEL like a Melee character you combo launch and edge guard the balls off of them. But vs the cast as a whole, he doesn't feel very "Melee". He's confusing and conflicting and just wants to Knee around the field like the rest of us but can't.