It's kind of weird. In the late game robots aren't useful for ground fighting anymore.
My understanding is that Gene Warriors are actually the best army in the game when properly optimized- you shouldn't make them for just any old species, you should have a dedicated "warrior" species that you've Gene-Engineered (and for best results, made into Battle Thralls) that you make your armies from.It used to be there was two tiers of robotic armies, robotic armies which unlocked with droids, and android armies which unlocked with synths and were pretty useful.
Synth armies were taken out of the game completely.
However, robotic armies are not that bad, just mediocre. If you need better troops, I would recommend going for xenomorph, which are better than even psionic/gene warrior armies. Only downside is horrific collateral damage.