The movement just isn't the same and that's what makes a lot of the game different from the others, the only character who is even close to the speed of melee would be meta knight, but other than that they are pretty different for being both smash games.
Hmmm... It is unfortunate that I cannot be of satisfactory, concrete input. I hope my suggestion to the contradiction of the 'aristocracy of smash competence' argument is at least partially relevant:
Fox is fast, but as a result, the Fox player himself/herself must be one step ahead of Fox's actions at close or medium range, or else he will get cramped out by higher priority attacks. That is, the player must think in dyads of steps, or Fox will prove easy to overpower. Because his attacks are so quick and have short hit windows and range, it is easy for the player to attack prematurely. This can be exploited by knowledgeable opponents. He works best as a get-in-get-out character, and does not do well without an immediate follow up to a previous action in tight spaces.
Falco, being much slower, is easier to control, and has greater hit consistency, but his attacks are of limited use when he is threatened from multiple angles in a short period of time without landing a hit.
Marth is challenged by the interference of his attacks (which additionally have short hit windows) with his mobility, forcing him to attempt multiple quick, short-lived strikes. This can be exploited by provoking him to rush from a predictable distance (which is especially good news since he is without projectiles or horizontal boosting attacks). He also has difficulty pressuring opponents that are calmly and methodically moving away from him.
Jigglypuff is 'squishy' and terrible on the ground, but excellent in the air, so with the right prediction and baiting methods, it can be 'roofed' in. Do not let it gain an altitude advantage, or its aerial attacks combined with its jumps, side special, and airspeed act effectively as a large, grounded character that has good reach and the ability to hit and run.
I am not sure what the best way of describing Pikachu's and Peach's shortcomings in terms of viability is, but for Captain Falcon, I refer you to
this.