Approach options:
PK Fire has too much end lag and is very risky. Fair has extremely small hitstun at low percents and is easy to shield grab out of. Dash attack doesn't always do what you want.
Kill options:
Back throw is great, but getting the grab is the hard part. PKT2 requires you to be a master magician to get someone to fall for this trap. PK Flash, only slightly useful for edge-guarding 1-track recoveries like C. Falcon or Marth, and still easy to dodge. Uair works sometimes. PK Fire/Thunder can gimp sometimes. Bair and Fsmash are probably the most practical kill options followed closely by back throw and Uair. Jumping off the stage to get a kill is very risky.
Movement:
Pretty much the worst recovery in the game and difficult to master the aiming. Probably the best thing about Ness as a character would be his DJC shenanigans. Pretty good wavedash. Medium speed. Nothing special.
Combo potential:
Ness can combo pretty hard, but usually only at medium percents. Combos usually consist of some combination of Uair/Bair, Up/Down throw, PK Fire/Thunder, and the occasional Utilt. Mix-ups require hard reads to be successful. Oh and I guess he has a Dtilt infinite on some pokemon stadium transformations.
Other stupidities:
Barely any CC options, horrible priority when being combo'd, pointless move (down-B), technically demanding, most of Ness' strengths require the opponent to make mistakes to be useful. Marth and Shiek are bullies, Falco is a moron.
Why play Ness?:
Because he's fun as balls to play. He's flashy and hype. Not many people know specific Ness MU information giving you the shenanigans advantage. DJC with Ness has so much untapped potential.
#FreeNess