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So I can interpret this to mean, is there a secret tech so bad that no race is able to use well. The answer is, no.

The other interpretation. Every secret tech has a race or multiple races that are best at it.
 
I forgot about syndicate
Happens to me. That and mixing up Syndicate and Assembly.

To your question, I did write a intro into Secret Techs, including a partial look at how races interact with them:
https://forum.paradoxplaza.com/forum/index.php?threads/secret-techs-101.1237908/

Some work better with others:
- Assembly being Cyborgs only work wonders with the Empire Upgrade for Synthesis Integration. Factions with Vehicles are also slightly better, but everyone can integrate every unit with the right mods. And there is that one OP one.
- Xenoplague works better on Amazons, Kirko and Assembly - compare to Mechanical heavy Dvar or Vanguard - as the whole rooster can get the mods and benefit from the ops. But as T1 and T2 units are viable options with mods, you can work with them on Dvar and Vanguard too.
- Compared to every other faction, assembly has issues using Promethean: They only got the basic Purifier and Arc can not be swapped to other damage channels. Meanwhile Amazons and Vangaurd get the repeating purifiers and have two channels that can be swapped into fire/burning (FIrearms/Laser and Biochem/Laser).

But at the end of the day, every combination is workable.
 
Xenoplague works equally well on everybody. The whole point of Xenoplague is that by grabbing those early sources of the debuff and unlocking the Pustule you can build up a bunch of side spawned units that you can have join your main stacks on the flanks. Pustules are not weak and you can literally stop researching Xenoplague techs after unlocking them and plague pods and get a lot of out it. I've done it. Did a Dvar plague game. I didn't advance past Pustules. Rushed to smart munitions instead. I used the Dvar explosive weapons from afar while diving in with expendable Pustules. Worked great.
 
Let us look at each Secret Tech and how they Interact with each race:
Kirko
Vanguard
Amazons
Syndicate
Dvar
Assembly

Celestian:
Kirko are propably the weakest. They got little use out of ST Melees, as they need the AA attack of the Frenzied more. And they already have the support/healer they need on T2. And regen replaces most of the Healing.
Vangaurd can use the Melee Option desperately
Amazons have a lot of shorter range, so SOM is a good choice
Syndicate can not use many of the bonuses on intendtured, but the Celestian Supports can help stacks without indentured
Dvar: I see no issue here.
Assembly: They got strong use out of the Shield and Stagger Resistance Mod, good melee options and do not need AA as desperately as Kirko.
So the "worst suited" are propably the Kirko. But everything is workable.

It works the same way with all others. I can definitely find a "weakest" Faction for any Secret Tech. But not a single tech without at leas 1 species that really works well with them.

Did a Dvar plague game. I didn't advance past Pustules. Rushed to smart munitions instead. I used the Dvar explosive weapons from afar while diving in with expendable Pustules. Worked great.
Dvar Xenoplague is a odd game. Normally Dvar are all about their Mechanical units. As theirs are not large targets, they are quite strong. But XP can not buff those.
However people underestimate trenchers with the Trench upgrade at their own perril. And as they can summon trenchers, they do not need the pustule spawns that badly, but it still helps.