I've been trying to think of things I haven't figured out yet.
One thing concerns models. Is there a good reason to develop a new model instead of building upon an existing one? I've only tried making a new model once, and it didn't seem to give any advantage over modification of an existing model.
When you create a new model, game rolls for structural design (leftmost column on the pic). IIRC, it's 84+d31, so the result is 85-115. It's a multiplier to all unit stats.
Base design is another multiplier. It's rolled when unit model is created. It's also rolled again, when the model is redesigned ("built upon"). The result depends on the field testing (i.e. how much you fought fith units of that model). Base design can only increase during reroll (lower roll is discarded).
Same goes for engine, weapon and armor design.
Base design and structural design are universal multipliers - they're included in all other unit stats.
For example, final attack = f(structural design)*f(base design)*f(weapon design)*(everything else).
A model with a shitty structural design always will be weaker than other models with equal inputs.
Since creating new models quickly becomes prohibitive cost-wise, you don't really need to rush to redesign stuff. There're some exceptions (like getting shitty infantry AND machineguns in the beginning... or having a shitty tank, since tanks are seriously overpowered), but generally you can hold your own and win with mediocre designs.
Also, structural and base designs don't depend on leader skills, it's a pure RNG. Leaders can affect only the speed of design research. So it's ok to put some retard in command of engineering if you're good on bureaucracy. The majority of leader skills don't do much.
Edit: I forgot to mention the following. Notice that GI 2 and Riflemen on the pic have the same automatic rifle. Also notice that Riflemen have better str.des and weap.des than GI 2. And notice that the difference in soft and hard attacks isn't that significant. Notice also that they're two times stronger than GI slugthrowers.
I.e. it's usually much more important to properly manage your wars on a strategic and tactical levels than try to optimize structural design.
How good is this on a scale from Alpha Centauri to Civilization 6?
It's closer to SMAC. But the game is pretty much in early beta.