It's really hard to make it so every character is good and has moves that are all usable, useful, and complement other moves from that character. Here's a list of examples of really good melee characters that have some insanely good moves with others that are either the most situational ever or are pretty much entirely useless: Peach dsmash to Peach utilt and ftilt, Fox usmash to Fox fair, Sheik most things to Sheik downb.
Usually the better a character is, the more likely it is that none, or few, of their moves are useless. Marth and Falco's most useless attacks are still really good. Sheik's most useless is changing to another character. That's kind of why watching low tier and playing lower tier characters is boring, you have to abuse their very few good moves while also dealing with their other vulnerabilities that would make any other character fragile. Such as how Mewtwo, Pichu, Kirby, Mr. Game & Watch all suffer from being incredibly light, but don't have the power to back it up like Jigglypluff, Fox, or Falco. Similarly how Mewtwo and Bowser both suffer from having massive character models that make them easy to pressure or hit with projectiles. At least with Project M they can change it at almost any time and can test things, unlike any other fighter game I've ever heard of or seen (afaik the dudes at Capcom at least try really hard, but it's not like they hold open betas or something). That's something I've always felt was really neglected in the "scene" by developers even though I know Nintendo doesn't care whether or not their game is competition friendly.
It's more so that they need to make it so the flow works. If every character is dying between 60%-90% when they aren't gimped, perhaps attacks need to have a little less knockback or something. This works from the opposite direction (the Brawl direction) where stocks feel "too long" and you're expected to die around 125%-150%. Ideally (at least I think so), not including the variable of stage sizes, characters of medium weight should be getting knocked into the blast zone from 95%-105% by attacks that are meant to kill.
I'm a professional game designer with many credentials, obviously. Next week I'll discuss why most games have horrible controls or interfaces that are clunky and make little sense (or whenever if anyone actually wants to read that).