Vel: snipes early game deny enemy adc farm, and when I hit lvl 6 pulling my combo off is garanteed to make the enemy back if not die. Die if my adc is nearby. Super strong, late game I 1 hit many mid champs (one hit combo that is)
Mundo: HP tank with a great regen and poke. What's not to love? ADC wants to fight? Go in and tank all the dmg and walk out. You won't even need to back due to the regen and get to stay and get xp+gold
Xin: You have sustain, you have damage, you have a gap closer and a way to isolate the enemy adc. You need synergy with your adc for it to work, but when it works its absolutely brutal.