Thresh Q is a 1.5 second stun as well as a displacement, Leona's E doesn't stun, Leona's E is a self displacement, Leona's E is a very small root. Thresh's E is a knockback which can't be reduced by tenacity and has a slow tagged onto the end. Thresh's E stun is longer than Leona's Q stun and it's ranged, Thresh doesn't put himself in the enemy team to lock someone down, Thresh's ult has a 99% slow which is stronger than Leo's slow when she doesn't hit the center of her ult.
Blitz pulls the enemy to you. In a teamfight he creates a pick where that a pull means instant death because you have no allies near you to help. Blitz repositions an enemy, knocks them up (which again can't be reduced by tenacity), and then silences them.
Both of them have just as much CC as Leona with less 'in your face' consequences. I find it easier to punish Leona for engaging than Blitz or Thresh.
Leona is also a primary tank. It's like calling Annie or old AP Nidalee as overpowered in some way (probably in terms of damage for both) as a support...they aren't really primary supports. In fact there are numerous people who carry themselves to high elo playing only Leona top/jungle.
I find her to be an obnoxious champion to go against because of how she's able to bypass minions.
She takes minion aggro by bypassing them and if you nerf her E to not work through minions you will probably inadvertly nerf her ability to waveclear in a solo lane as well as to help her ADC last hit in a duo lane as going through a wave with her E helps clear.
I find her to be an obnoxious champion to go against
And personally as someone who is 5/2 with Leo but also 4/1 with Thresh and 2/1 with Janna (aka Leo's 2 big counters other than Morgana) I don't find Leona to be OP or bothersome. She' only bothersome if I'm playing a squishy support with low CC aka Raka (or Sona). Nami bubble, Lulu's entire kit, Janna all don't bother me too much against Leona.