oh they're definitely op at 19, but they actually have counters at 19 (especially in arenas). you can run them out of mana, you can snare them and focus them, you can kill the pet VERY quickly, you can play los games etc. they can be neutralized, and they can be beaten - it's just not easy.
at 29, hunters don't have any counters. it's one thing to be overpowered, and another thing to have literally no counters. big difference!
at 29, you can't run them oom because of viper, you can't focus them because of disengage, frost trap, and cheetah. you can't really play los games because all they have to do is keep instant aimed shot up once every 10 seconds and you lose the mana war.
and of course, all of these factors get exacerbated x10 when you step out of an arena into a battleground without mounts where hunters reign supreme.