Ruinous Ultimatum is a terrifying one-sided boardwipe, while Cruel Ultimatum hurts the opponents in a myriad of ways as well as benefiting its caster and netting massive card advantage
For example, hunting up Underworld Breach, Lion's Eye Diamond, and Sevinne's Reclamation ensures you have a Breach line, no matter what your opponents give you
They are strong beyond belief
It won't copy any counters or stickers on that creature or any effects that have changed its power, toughness, types, color, and so on, and a creature becoming a copy doesn't remove any such counters or effects that are already applying to it
It obviously has a home in blue control decks, but I dont think they will want to play the full four copies of the legendary enchantment, even if it isnt too bad in multiples, as you can have one transform into Azcanta, the Sunken Ruin while the other stays on the enchantment side