See Vitals-Curser. Changed Shootout to make Discarding optional, so to avoid forcing discard and thus breaking color pie.
I believe optional discard for the mechanic does not break the color pie, since the player has the choice to lose the shootout. Should no player discard a card or if two or more players discard cards of equal CMC, then there is no winner of the Shootout.

Right now the shootout does not seem worth it. Card disadvantage t get a spell exiled? And if I ever get an effect out of a shootout I actually would want, then my opponent may still cancel it?

Reads bad in the rhystic vein to me.

That's a fair assessment. In a vacuum, unless you wanted to fill up your graveyard, you're facing a card disadvantage just to keep something out of a graveyard to be reused. If I don't make heavy use of the graveyard for the set, this example may not be worth including. However, if your opponent chooses discards a high cmc to win the shootout, there's a good chance they're paying a hefty price for something they'll miss. But, if I don't make high use of the graveyard for the set that this mechanic would appear in, this individual card may not be worth including.

I probably need to add words to make the effect work better. Something like "Each player may exile a card from their hand face down. Then, each player reveals the card exiled this way and puts it into their graveyard. The player who exiled the card with the higher converted mana cost wins the Shootout."?