I've been playing GC3 for a couple of weeks, using (and occasionally adjusting) a synthetic custom race. There have been issues with this race not working as expected. I could work around these, but with the latest patch(es) more severe issues turned up that are clealry not intended changes, and effectively break custom race builds as a whole!
FYI, my copy of GC3 comes from GOG, and I've set the GOG client to auto-install updates. I have no DLCs or expansions, only the base game.
The issues I noted so far in no particular order:
1. synthetic is no longer listed under custom race abilities. However, when I create a new custom race, and later get to select a Civ for a new game, the properties do show the synthetic trait, and that race is indeed not affected by pop growth and does not rely on farming.
2. When I try to edit an existing synthetic custom race, I see one free ability point. Since there is no synthetic ability, that is obviously where that point comes from.
3. The new 'hidden synthetic' custom race doesn't grow or eat, but it can't colonize barren/toxic/radioactive worlds without research! This used to be possible with my older synthetic race build.
4. In none of my games, researching +synth pop cap techs did achieve anything! Nor constructing the +pop cap building. The planet pop caps always remain the same, and I can not assemble more pop once the planet pop cap is reached.
5. Although I've read older comments to the contrary, taxation does affect productivity for both the old custom synth race and the new ones. Morale too, but apparently that is intended.
6. Probably not synth-related: when I load the maxium available population from a planetary system onto a colony ship, rather than leaving each planet at 1B pop, one of them will suddenly show 3-4B instead - you can effectively create some bonus population in this manner at no cost other than colony ship movement points! You can test this at turn 1 with the initial colony ship, but you must send it to the shipyard and then relaunch, not to the starting planet.
I should add that all my custom race builds used the prolific ability along with sythetic (while it was still a selectable ability), and that I use maxed-out multiple-module colony ships to settle new planets. This may not cause the issues, but it makes them more noticable.
7. in my current game, I've noticed that the Terrans offered the first +synth pop cap tech for trade. I have set the game to 'no tech brokering', so they shouldn't be able to trade a synth tech. Not to mention they shouldn't bother to either research it, or trade for it!?