I believe this claim was made due to Ness' ability to kill reliably (and relatively early as well) and how he can rack up damages like if it was nothing. His aerial game is very good too.
The hilarious irony of such claims is the complete and utter hypocrisy of it all. Especially if you consider just how many of these same people who salt about Ness, main or secondary someone infinitely more cheap in BOTH departments like Shulk, Yoshi, Rosalina, or "Trollager"; or Diddy Kong before he got nerfed into oblivion.
Shulk can, has, and will, rack up incredible damage in the blink of an eye with Buster, then switch to Smash to make virtually all of his moves reliable kill options - which is almost assured given his incredible range unless you stall it out or turtle. The only real blessing is it can backfire if you're a good enough player who's experienced in punishing Shulk.
Yoshi can inflict up to a mind-blowing 34% on average with a single dAir! That's about 1% shy of a fully charged PK Flash iirc! He also has quite a repertoire of reliable kills and perhaps the best spike in the game that's aided by a super-armored second jump. Add to that Yoshi's low lag which makes him rather difficult, if not near impossible, to punish when in the right hands to the point that you just have to slowly and painfully chip at him until can go for kill with bThrow.
Rosalina... one word, Luma. 'Nuff said. I'm not even going to get into how absurdly fluid and powerful she is, or the fact she's like a glorified Ice Climbers but without any of the drawbacks.
Villager aka Killager aka Trollager can rack up damage fairly quick as well by camping and spamming SH nAir and Lloyd Rockets in alternating sequence; can gimp and punish hard with Pocket, has RNG for uAir and dAir that can be devastating; and again, also has reliable kill moves too. I'm just glad his recovery took a hit so he's not as obnoxious to deal with.
Yet these are the same people we're stuck hearing whine ad-nauseam about how "cheap" and "broken" Ness is on a consistent basis. One person from chat I know personally salts insanely hard about it when he's using Shulk and Yoshi of all characters. Again, as others have said on these boards - Ness has a bad neutral game, bad approaches, and an easily punished recovery. He's balanced, not broken by any length, but regardless, we use him because we love him and the series he came from. Heck, I even mained Ness in Melee without a care for his weaknesses, so I'll be damned if anyone accuses me of using Ness just to win or because he's "broken".
If you can't handle Ness, or feel the urge to whine about him, I have five words for you: Learn how to play Smash! I'm sure the second one of us dethrones ZeRo with our beloved main, he'll cry and cry on YouTube about how badly Ness "needs" a nerf... it's sickening. But alas, given Ness' weaknesses, being able to even make it far enough to face ZeRo might prove to be quite the undertaking, yet Ness is "broken", okay, sounds legit.
EDIT: Corrected typos