High risk High reward champions

I wouldn't call either of those two high risk/high reward.

I would think "high risk/high reward" champions are those like Katarina, who don't have a lot of build path in the way of defense but still need to be in melee range to be effective. It's a gameplay pattern that appears mostly on Assassins, I would think, and less on Marksman, Supports and Mages.

Hmm. High risk...

I think Malzahar can count in this. He has a big risk in losing mana a lot. But his attacks are super strong. Though there are ways to overcome this ( a little).

Gnar has proven to be of high risk. He has literally low to no armor, magic resist, or anything by that matter. And takes a lot of skill to master but is good tank and team game player.

Veigar can also be another as he is super easy to kill. But he can deal amazing damage.

Kennen is high risk, high reward. He needs to be up close and personal even though he's a squishy mage, but if you pull off his combo properly you can singlehandedly turn a team fight around.