Karma's damage once morellonomicon is finished is quite high, especially when a Q+R is hit on a adc or mid. In a team-fight, Q+R's after hit circle is very hard to see, this makes the damage very high and will shred through any squishy opponent in your way. E+R shield is stronger due to the extra AP which helps out all allies on your team.

I generally stay clear of this pick if the other team picks a burst assassin jungler. This is due to the fact that the assassin junglers make a very quick and easy snack of Karma any time they see a opportunity to. I also do not pick Karma when the enemy team has a Blitzcrank because of the very difficult laning phase which consists of constantly attempting to constantly dodge hooks.