@SavatoThe Doomsday gene is a self-adapting self-reprogramming form of DNA. It's implied it doesn't even use amino acids like our DNA does. It's quite easy to 'Doomsday' things, but keep in mind that the gene will keep adapting and evolving. Meaning that a sectoid given it as a 'boost' would start becoming more and more Doomsday-like, until it turned on the Ethereals.
The only controllable Doomsday clones were massively weakened versions. Darkseid's army of Doomsdays was so weakened that Batman killed one with an axe, but that's the only way he could control them. Brainiac, a massively psionic death-robot, was able to create a Doomsday clone that he inhabited as a body. However, the body eventually became immune to him, and he was nearly destroyed.
Doomsday DNA makes excellent weapons, but they inherit their papa's uncontrollability.