I summarized it at the end so if you want the short version, just scroll to the bottom.
After playing games against her and as her, I can honestly say she needs a nerf. Not towards her damage even though she does a lot of it, but towards her CC. Her abilities have such a low cool down especially on her Q and R that it is ridiculous. Her Q doesn't stun you for a long time, but it's enough to make killing you so easy, her ADC can just auto attack with no passives or abilities and still kill you and she's able to use it enough in a fight to make you borderline useless. When she builds CDR, that 5 second cooldown turns into a 2.75 CD with 45 CDR. You don't think that's excessive especially with other champs who have CC abilities that deal more damage? Good Leona's know how to cycle through their abilities spacing out their E and Q use so they can make spamming CC even more frequent.
Her R CD is too short and is too effective in a teamfight. I've built CDR on her and her already short ult CD became even shorter allowing me to use it one fight after another fight like I never used it to begin with. Being able to use it as often as you can for how effective it is like giving Sona or Nami's ult Blitzcrank's ult CD. Either nerf the cooldown on her ult so she isn't able to use it almost every fight no matter the window or nerf the AOE on it so it isn't hitting everything in lane. It's like having Ashe's ult but instead of stunning one champ, it stuns multiple champs and slows everyone else.
I understand she's an engage heavy champ and I understand the basis around her kit is to have as much CC as possible, but she does her job too well for how easy she is to use and how spammable her abilities are. Even Alistar, who has the same champ concept which is heavy CC engage, has long cooldowns on his CC abilities. Nautilus has long cooldowns. Leona isn't overpowered. I'm not arguing that. I'm arguing the fact that her abilities are too spammable making her kind of busted.
I understand post level 6, she has to all in when she engages and can't really get away, but her spammable stuns make it very easy for your ADC to get a kill unless they have a disengage support or her W, making her more tanky than she already is, gives her enough survivability to sustain enough to leave the fight with more health than the enemy. To put it simply, she's tanky enough to all in and still be fine and she does enough damage by herself to almost guarantee a kill if her ADC helps.
My proposal is to increase her level ult CD to 120 or make the AOE on her ult smaller and increase her Q CD to 10-14.
Her Q CD is too short, her R CD is too short and her R is too effective. Even though she is dealing more damage than she needs to as a CC tank support, the fact that her CC is so spammable is what makes her seem overpowered despite her not being overpowered. Increasing her Q CD to 10-14 and increasing her R CD to 120 OR nerfing the AOE on her ult would make her a lot more balanced and fair to play against. She would still function the way she is supposed to by locking down everyone, but she just wouldn't be able to do it as often and freely as she is now.