One thing a lot of people don't fully realize about voidtech is that echo walkers are the only unit in the entire game that can summon a unit that retains abilities granted by mods. What that means, is that if you put brimstone missiles and jetpacks on your echo walkers, both the clone and the original get to use them. This can get hilarious pretty fast, and there's some wacky degenerate combos you can pull off in empire mode with other secret techs, like malware demon swarms, or phoenix bomb+brimstone missile for pure artillery walkers. It's a fun gimmick, and surprisingly effective, especially since the new vanguard racial building encourages you to spam units.
Synthesis doesn't really have any flashy combos like that. It's own units and operations are pretty situational, but it has some nice mods and some solid doctrines. Plus, if you're having trouble with mechanical units, you can start actually using your secret tech units and hard counter the shit out of them.