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.