Why Rebels are so BROKEN? (What about the rest of the characters...?)

It only matters if you're bothered about winning.

I'm not saying you shouldn't care about winning, that's up to you. Play the game how you want.
I prefer having fun with a team I like playing so I mostly use Jedi. I get whooped a lot in arena (especially as the AI refuses to use my team properly), but as long as I get in the 200-500 bracket, I'm getting some crystals and 700 arena points, which is enough for me.

If you want to win - farm the meta team. I think it's kinda boring and unimaginative, but it's the way to win.