People have been saying that Pikachu has one of the best disadvantage states in the game ever since the inception of smash 4. It was kind of true in 4, but his disadvantage state got a lot worse going into Ultimate.
Quick Attack as a landing option is overrated; it has a lot of startup (more than many smash attacks) and there are only certain heights from which it can be used to land safely. During the animation Pikachu's hurtbox expands significantly in all games (this expansion seems to have been toned down a bit in Ultimate, admittedly). Against slower characters like Incineroar it's a great tool for getting out of the juggle, but against anyone with sizable disjoint and/or speed the other character can dash around on the ground and wait for Pikachu to commit to something. Platform canceling made QA landings much better in S4; in Ultimate, Battlefield is actually one of Pikachu's worst stages (partly due to tjolt interactions), so you almost never want to go there and abuse all of the QA landing spots. Additionally, at low percents, QA won't even combo like it did in S4, so you don't have much to be afraid of getting hit by it, you can feel free to take a dash-in guess to his direction if you don't feel confident in reacting to the animation and then f-smash him for your trouble.
Despite having decent air accel, Pikachu has really poor max air speed and poor hitbox coverage on the way down. He doesn't have a Palutena n-air or a Lucina f-air to cover after accelerating to one side, and he falls somewhat like a floaty and it's pretty easy to get under him when he's landing. This was even true in 4. Megaman up-air used to be one of the sickest tools against Pikachu for this reason in that game. Back then, though, Pika did have his f3 strong-hit n-air, which was a strong deterrent to aerial pursuit. His flashy new combo'ing n-air doesn't have the same escape power. The stronger up-air is definitely a buff to his landing options, but at the same time he also lost airdodge into pancake b-air, one of his strongest tools against fast grounded characters who didn't have to worry about QA like Sonic.
Overall he's much, much worse at landing than he was in S4, and in S4 he already wasn't that good at it once you learned to be patient and didn't overcommit to the juggle.
It's also actually not that hard to edgeguard Pikachu if you're well-versed in the matchup and have a decent lingering tool; there are a lot of angles from which he can recover but QA's startup makes it so that you can always get a rough estimate of when he's going to land, and you know that most of the time he's going for the ledge. Ike eruption used to eat Pikachu's recovery for free in 4 in so many contexts (granted, Ike's eruption ate a lot of recoveries for free, but still--for something that was cited as "the best recovery in the game," that's a pretty severe shortcoming).
When Pikachu is in the corner, he does have some pretty good options for getting past you, which is one of the few often-overlooked merits to his disadvantage state. And it's probably fair to say that his disadvantage state is in the upper half or upper third of the cast. But it is rather overblown and not free in any sense.