I've done some more looking, and it turns out Synthetics are not the only ones with a mostly absent Planetology branch. Silicon based races have the same problem.
Also, the Synthetics' Xeno Commerce branch is mostly absent. It only has Xeno Commerce and nothing else (other races seem to be fine).
Finally, for Synthetics and Aquatics only, it seems that the Dimensional Fatigue tech (Engineering, top branch) offers no improvement whatsoever while for other races it offers what looks to be a starbase improvement.
I learned all this by comparing the tech trees of the Yor, Onyx Hive, Slyne, Torian, and Iconian factions.