At worst, you get an explosive turn and one creature gets a bunch of tokens
For the cost of a card, you can counter an opponents Unearth, putting you up two cards (while both players have spent a spell, you are up a Zombie and your opponent is down the Rat effect)
Too weird without it mattering
Greens ability to ramp will make the mana cost not much of a problem, and the loyalty abilities are the bomb
Josh Lee Kwai highlighted a five-color Nekusar the Mindrazer deck very early into The Command Zone podcast run, and I discussed aspirations of building around The Gitrog Monster in a four-color deck in the last few months
Even if the caster has a weak board, Ruinous Ultimatum is still worth casting if they can't wait for another, cheaper board wipe