As it is now aliens are only of any consequence in the early part of the game. They should become stronger over time, perhaps in line with the faction that has the highest harmony affinity, as the dna from all the experiments and creations leaks into the indiginous spieces. Not that they should become as strong as the true xeno (whats their name huge manticore thingamajings?) that requires 7 xenomass to build? But they should definitely become stronger, perhaps upto 60-75% strength of the highest upgraded solider unit.