Karin did not heal tsunade FROM CUTTING IN HALF she healed her from loss of energy .
Tsunade was weakened and had no energy to the point her old form is showing .but karin healed her from that (almost any medic can do it btw even tsunade eating food had the same result)
THE ONE WHO STICKED TSUNADE BODY WAS TSUNADE HERSELF AND KATSUYU KARIN HELPED GIVING TSUNADE ENERGY CHAKRA SO TSUNADE CAN HEAL HERSELF AND SUIGETSU HELPED HOLDING TSUNADE BODY SO TSUNADE CAN HEAL HERSELF .THAT DOES NOT MEAN SUIGETSU AND KARIN ARE THE BEST HEALERS
Also theres a difference between tsunade case in pein arc and the fight with madara
Tsunade did not heal fast
Because she used her byakogu seal to the whole village and the byakguo gone
But in the fight with madara her byakguo seal is still there
And something most people don't understand karin didn't stick tsunade body she gave
her chakra recovery after she lost a lot of chakra .
Tsunade than uses the byakugo seal after her chakra is back and katsuyu (not karin
) stick tsunade body . [as you can see katsuyu is standing on tsunade belly)
In fact katsuyu was able to heal tsunade alone with tsunade left chakra but tsunade said that she should heal the kages instead .
And that made tsunade healing prosess slower .