The one thing these really lack is cohesion between the color intensive effect and the Nephilim-effect. Horrific Ultimatum could benefit tapping into black's ability to tutor (e. g. for an instant or sorcery to benefit from the other half of the spell's effect) or Sins from the Past; Formic Ultimatum could grant haste to the tokens to partake in the attack trigger etc.

As long as the color intensive effect and Nephilim-effect of each spell don't synergize positively with each other this cycle falls short of its potential.