After playing for a bit I decided to look at a bunch of the secret tech units and unlocks. I have seen some patterns with them and some race mixes are better than others.
For example, any race that gets weapon technologies that match with the damage channels of the secret technologies will naturally have more mods they can use on those secret technology units. Some races aren't as good at using secret tech stuff in general due to a lot of their units being non-infantry, see Amazons, Vanguard, and Dvar. Certain secret tech status effects synergize better with certain damage channels (voidtech's dimentional lowers kinetic damage resistance), while others are universal (xenoplague's parasitic infection just needs to be on the unit).
Another is something that I think the devs might have actually run out of time on. The amazons in particular have a unique unit variation for the promethean, voidtech, and celestian tier 2 units (their psynumbra units do get unique abilities though). Most of the other races just have a default unit with maybe slightly varied body shape. Dvar do have unique variants for voidtech and synthesis though. They probably intended to give all races these variations, but ran out of time to do so, mostly finishing only the amazons plus a few scattered others.
Some of the other secret tech units do get a few slight variations based on race. For example, melee races will get their tier 1 melee unit ability on a tier 2 secret tech melee unit. Kir'ko get frenzy and Assembly get scavenge. A few units get a racial expertise +10% damage trait (D'var plasma weapons, Kir'ko dark psionics). Sometimes vanguards will get grenade-like abilities instead of the normal ones. A couple of the syndicate units are indentured with the control collars built in.
Funfact, no one actually gets direct synergies with xenoplague units, since there are no unique racial variants for them. The most you get are unit mods that happen to fit with them and mods from the tech that fit your units. Funnily enough, one of the most important mods for xenoplague are the plague pods which can be applied to any non-animal. This means the amazons are arguably the worst combination with them due to so many of their units being unable to use plague pods to cause parasitic infection.
For example, any race that gets weapon technologies that match with the damage channels of the secret technologies will naturally have more mods they can use on those secret technology units. Some races aren't as good at using secret tech stuff in general due to a lot of their units being non-infantry, see Amazons, Vanguard, and Dvar. Certain secret tech status effects synergize better with certain damage channels (voidtech's dimentional lowers kinetic damage resistance), while others are universal (xenoplague's parasitic infection just needs to be on the unit).
Another is something that I think the devs might have actually run out of time on. The amazons in particular have a unique unit variation for the promethean, voidtech, and celestian tier 2 units (their psynumbra units do get unique abilities though). Most of the other races just have a default unit with maybe slightly varied body shape. Dvar do have unique variants for voidtech and synthesis though. They probably intended to give all races these variations, but ran out of time to do so, mostly finishing only the amazons plus a few scattered others.
Some of the other secret tech units do get a few slight variations based on race. For example, melee races will get their tier 1 melee unit ability on a tier 2 secret tech melee unit. Kir'ko get frenzy and Assembly get scavenge. A few units get a racial expertise +10% damage trait (D'var plasma weapons, Kir'ko dark psionics). Sometimes vanguards will get grenade-like abilities instead of the normal ones. A couple of the syndicate units are indentured with the control collars built in.
Funfact, no one actually gets direct synergies with xenoplague units, since there are no unique racial variants for them. The most you get are unit mods that happen to fit with them and mods from the tech that fit your units. Funnily enough, one of the most important mods for xenoplague are the plague pods which can be applied to any non-animal. This means the amazons are arguably the worst combination with them due to so many of their units being unable to use plague pods to cause parasitic infection.