There is an egregious exploit with Valki, God of Lies; while on the battlefield, and with a creature exiled with it (maybe even without?), you may choose X=0 to consistently activate it to your advantage and stall out a game.

Please see the attached screenshot; to the best of my knowledge I was in a winning position, but with very little time on my clock. My opponent then proceeds to keep activating Valki, with no way to interact, or benefit to his board state, until the client forfeits me from the game.

This is unacceptable, and would not be allowed to happen in a paper game.