I honestly can't recall anyone complaining about the Witcher 3 being too long, nor can I imagine a reason why someone would complain about a story-heavy game being too long.
For me, it's less that it was long, so much as the pacing was rather wonky at the end.
Basically, that game, after the Battle At Kaer Morhn, I thought was wrapping up at that point. Everything about that mission just screamed "final or penultimate" mission, what with it reuniting all the allies you made and showing down at where the game begins, and after it was finished, I thought that the game was gonna wrap up pretty quickly soon. And honestly, if it did, I think I would've had a far better overall impression of the game.
Except, no, that's not the last mission at all, for there is in fact still 10+ hours of story left to go through. At this point, the game's momentum comes to a screeching halt as you then have to do all these extra story missions that, if I'm being perfectly frank here, I found just needless padding before you can actually get to the last mission. There are also no more new locations aside from one-offs during one of the story quests, which also makes the last 10+ hours of the story feel like they drag on even more. It really put a huge damper on my enjoyment of the game, as it was paced so perfectly and was flowing really well until that point, at which point I was just feeling like Tom Servo from that one MST3K episode, screaming at the screen for the game to just end already.
So yeah, it's not really the length that's the problem, but more how much the pacing and momentum takes a nosedive at the end.
Which is why CDPR's comments regarding the length in Cyberpunk 2077 are very encouraging to me, as it looks like they learned from their mistakes with TW3's ending.