Also a minor point but gene warriors also don't have the problem of being countered by robotic armies.
No army counters any other army. Here's a table showing in a 1v1 fight, how much damage as a % of your HP the enemy army would do by the time you kill it.
As you can see, if you ignore Garrison and Defense, you roughly come up with a couple of tiers in terms of strength:
Tier 1 - Assault & Slaves - Slaves are much better once you include their significantly cheaper build cost and maintenance. In fact Slaves almost come out on top over everything when you include maintenance and build costs.
Tier 2 - Clones & Robots - Clones are vastly superior once costs are included. Since they cost the same as Assault troops you should replace them if you care to. As pointed out before once costs are in Slaves win out, since while they should perform similarly in terms of maintenance, a Slave army would be 1/2 the mineral build costs. Because of the costs in maintenance and build costs Robots are just bad, almost unarguably the worst army unit.
Tier 3 - Elite Guard, Androids, Psionic, Xenomorphs - That's their order of strength. Psionics will beat out Xenomorphs but that's the exception. Since Xenomorphs cost less in build and maintenance than both Androids and Psionics, there's no reason you should be building those. Psionics will heavily beat out Androids if you look at maintenance but Androids are significantly cheaper to build. Elite Guard are the best you'll have until you get Xenomorphs, course Slave armies are still cheaper to build in terms of minerals...
Tier 4 - Gene Warriors - Exterminate! Exterminate! wait, what? did you expect an Exterminatus? Everyone knows Daleks are the perfect analogue of a living being and the superior race.
Tier 5 - Titanic Beasts - get them when you can and what monster wouldn't use Cthulhu for menial tasks whenever they could?
Of course when picking weaker army units you will always have a higher statistical chance of accidentally losing an army. But lets face it, as you can see Slaves are the Ninjas of Stellaris. Just keep throwing them at the problem until it's fixed.
Overall in a SP game the costs of armies are negligible (a gene warrior costs what? around what a destroyer costs) So even if you are building a 100 of them you can really just build what you want.