I’ve won with most characters in many different SF games. It isn’t entirely true that Ken or Ryu are overpowered, although in Street Fighter II Turbo The New Challengers, Ryu is disgustingly overpowered. If you can clear the game with him, don’t even think it’s because you’re good.
Otherwise most characters have the potential to be good if you master their move sets. It helps that a lot of the moves are the same for many characters. If you’re good with Ryu, then you can probably clean house with Sagat, if you’re good with Chun Li, then you can learn Guile or Dee Jay. You have to be a bit more than a casual SF player to know what I’m on about though.
Lastly Jerv comes up with a good point, devs have their favorites. When fighting against the computer, Ken, Ryu, Blanka and Sagat are always tough to beat. This is due to programming however, and has nothing to do with said characters being overpowered, being that you’ll never be awesome with those characters as the computer is, unless you’re some kinda SF god. For example, Dhalsim could be impossible to beat if the devs wanted it, and that has been proven in the Alpha series. Yet, he’s reputed as being the shittiest SF character ever.
There is truth to some character balance issues, but there are SO many SF games out there that you’ll have to bring up a specific game if you want to discuss this accurately.