Leona doesn't make sense to me as a competitive pick.
With a life lead, she functions okay as a defensive character. But I would say that Ralf, Ash, Saiki, and King (at least, maybe others as well) do better with that playstyle because of better projectiles and farther reach in the zoning game (King's Tornado Kick, Ralf's Gatling Attack, etc).
Without a life lead, She needs meter to function well as her combo damage is below average without it. Her projctile's slowness makes it impractical to use in an approach. Her unseeable 50/50 is strong, but it wont kill someone unless you land it two or three times. Plenty of characters can kill you in two or three hits with 4 bars, so I don't see a significant advantage there. Lastly, her HD combos are a bit more difficult than those of other characters, although technically this is a non-issue with enough practice.
In either case, I don't see a reason to play Leona over another character, currently. Hopefully new tech emerges.