I'm testing assembly right now as they are by far my favorite, here are my general results.
Voidtech is the best secret tech for assembly with synthesis a close second. Promethean and Psynumbra are both pretty bad on assembly overall. Xeno can work, but it had an inherent lack of synergy, that being said it is already a snowball tech and assembly is a snowball faction so it's really an all in strategy. I haven't tried celestial assembly, mostly because it seems like it would just be terrible. I will try it though to confirm.
If I were to rate best to worst:
1. Voidtech
2. Synthesis
3. Xenophage
4. Promethean
5. Tie between celestian and psynumbra. (As I haven't tried celestian yet, I'm not sure which is worse).
So after more testing here is my updated list. I'm 99% sure of these rankings. I have roughly 55 hours of testing Assembly secret techs and strats done so far.
High Tier:
1. Assembly Voidtech (best all around)
2. Assembly Synthesis (best versus heavy mechanical armies and therefore best against certain match ups even stronger then VT. However it's not very good versus biological armies so I give the overall edge to VT but it's close.)
Medium Tier:
3. Xenoplague. (Assembly are a snowball faction, Xeno is a snowball tech, the majority of assembly units can use Xeno mods, and there are some assembly mods that work well with the Xeno playstyle)
4. Celestian. (Heavy focus on defense and survivability on a faction that already has a lot of strong defensive options, however there is a lack of overall synergy that makes it worse then the former 3 combinations.)
Low Tier:
5. Assembly Psynumbra (a distinct overall lack of synergy, but the assembly psy units are good and allow for some extreme fodder strategies. This combo gets a low rating due to just how many hoops you need to jump through and the specific playstyle you need to adopt, which is using your psynumbra units as effective suicide bombers to absorb fire and spawn into chorus while you clean up with your assembly units)
6. Assembly Promethean. (Huge lack of synergy and assembly specific prom units are nothing special, can't use the majority of promethean mods to any real effect on assembly units)
All of that said, no combination is bad or unplayable or you can't win games with. This is a relative ranking of the ST combos with Assembly so remember I am not comparing them in a void but in relation to each other. If your response to a list like this is to say "well I've used x combo you listed as not the best and I did well with it" that's fine and totally expected. What I would respond with is to try the combo that I list as better (if you already havent) and see if you still come to the same conclusion. For instance if you have only tried celestian and promethean assembly, then in comparison celestian seems really good! Now go try voidtech or syndicate and see how much stronger those are then celestian!