The claims are such hyperbole; people whine and salt way too hard about bThrow and PKT2 primarily. It's kinda sickening - and that's coming from someone who has fought his share of Ness's and actually enjoys fighting Ness (both without Ness, and in Ness dittos.)
Let's get a few things straight:
1.) Characters are allowed to have kill moves. In Training, bThrow, at the ledge of Final Destination, doesn't even kill Mario until 87% exact, without DI. fSmash uncharged won't kill from the center of FD until 78% if sweetspotted at the tip - otherwise it kills from the center of FD at 103%. Mind you, that since Training adjusts both players percentages, you'll have to factor in that the rage mechanic is helping too and that these moves probably kill a bit later in reality.
That being said, don't even get me started on how early Ryu can kill if you're using the Street Fighter input for Shoryuken. Also, don't get me started on just how fast Shulk can rack up damage and kill with Buster into Smash arts respectively, along with the ridiculous range he has - especially in comparison to other sword users.
2.) PKT2 is move that kills Mario from the center of Final Destination at exactly 40% without DI at the absolute earliest (according to the findings of both myself and the smashbros subreddit irc chatroom, which I admit is a bit early - perhaps 50 or 60% would be more fair), BUT ONLY if sweetspotted by using it at close range (roughly a little less than a quarter of the length of FD or closer). The knockback reduces considerably the further Ness has to travel, and can even be knocked out of it after a certain point. Last but not least, the move has roughly a 1.5 to 2 second long audio and visual telegraph (longer if you're attempting mindgames) where Ness is reduced to a sitting duck for punishes - if you can't react to that, or if you set yourself up to get punished by it, then you deserve to get hit by it.
3.) PK Fire is easily escapable. You can DI out of it, perfect shield any of the rapid hits (once already caught in it) then roll out of it, and even jump out of it, all with little effort; making it near useless against any mid to high level player. I'll use it once near the beginning of the match to see if they know how to escape it, and if they do, I don't even bother to use the move offensively for the rest of the match, and will instead use it only as a defensive measure to shut down certain combos and approaches.
4.) Yoshi. Little lag, very small punish window, super armored recovery, and oh, let's not forget his dAir can do in the upwards of 34% in a single use if done right. If that's not broken, then I don't know what is.
5.) Ness can be gimped to oblivion and back by virtually everyone. Villager and Rosalina come especially to mind.
6.) Ness, is primarily known in Smash, to my knowledge, for his punish game. Others claim that Ness has a terrible neutral game, bad approaches, and easily punishable recovery - that said, Ness' killing ability compensates for that. Take that away, and he doesn't have much else going for him.
7.) As virtually everyone else has pointed out, people with planks in their own eye are so quick to point out the splinter in another's. There are quite a few characters that are far more broken / cheap / etc.
If you ask me, Ness is pretty balanced at best, slightly above average at worst, but regardless, he's nothing that any mid level player or higher can't handle; and has a decent amount of counters.
EDIT: Corrected typos and incoherent sentences. I really shouldn't type long posts at 5am...