For me, hearing people say Ness and Lucas have very little differences is like hearing people say Mewtwo and Lucario have very little differences. I played as both a lot in Brawl, and I had to use both differently.
Edit: Lucas' b-air can spike if timed and positioned correctly. Ness' just goes in one direction. Lucas has an extra recovery option with tether. Wolf's reflector comes out slower but the projectile is reflected faster. Lucas' PK Fire travels further and pushes opponents back while Ness' may trap opponents who don't know how to DI out of it. Lucas' Psi Magnet may do damage while Ness' can't.
If you're playing as them, then of course you'll be more in tune with their differences. But your opponent, does he even care which one you choose?
Bair... It has short reach, correct? Ness's Dair can meteor. Your opponent doesn't care what buttons you press, just the results.
Last I checked, it wasn't exactly something to put much trust in. True enough to the game, him?
And more powerfully.
Probably the most significant difference.
Like anybody will ever get hit by it outside of some glitch. It's considered to be useless in SSB4, even.
I just see all these tiny changes as wasted effort. They could of put all that work into giving them something people will notice, something that'll make people treat them much more differently. You'll see significantly different matchups for Pit and Dark Pit, and there's only three moves with any difference at all.