The Legend of Korra

 Spoiler

>Because Korra learning to Airbend wasn't the premise of the show. She was only shown actually trying to Airbend like... twice. Yeah, it's not the best time to whip out that she can do it, but it's not like it's Energybending where it was just like "OH LOOK. AANG HAS THIS NON-LETHAL METHOD TO DEFEAT OZAI. HOW HANDY. LEARNED IT JUST IN TIME TOO."

Actually, if I remember correctly Korra never once performed Airbending until the last episode. In the Pro-Bending episode she started fighting like an Airbender, with fluid movements and the like, but I don't remember her ever actually Airbending. That's why she was so surprised that she did in the last episode. It's possible that the Bloodbending reversed the bending abilities in her (IE, she could Firebend, Earthbend, and Waterbend but couldn't Airbend, and it reversed). Don't ask me how that's possible, just theorizing.

But what happened to Zuko's mother is just a general wondering in the world. How bloodbending -> taking away bending is just a what. Not to mention they did acknowledge that everyone wants to know what's up with Zuko's mom anyways. I'll join Corn in the explanation that bloodbending is just "that strong."

While I will agree with the first part, it doesn't change that people will and are wondering how it could do that, something that will probably never be explained (Just like Zuko's mom never was).

Welp I gotta say Polantaris blood bending is pretty OP, since only the Avatar and other bloodbenders can stop it, and now it can take away someone's bending.

At least the only three known "strong" users of it are presumably dead.

Strong abilities against one person and being able to use it against hundreds is two different fields of expertise. A good example is Lightningbending. It's very strong, but only is able to target one person. Yakome and family's Bloodbending can magically fill both fields. I still call bullpomegranate "OPness," so to speak.