They grow their population must faster and their power graph increases dramatically over their non-Synth counterparts.
The reason is because the AI gets massive production bonuses in insanity, and they use it to build pop. And so they develop their world much faster in comparison.
As a result, for a challenge I change all the AIs to synthetic race with Yor tech tree.
I wish someone would create a "synthetic mod," that changes the AI to synthetic but combine it with their unique tech tree. For example, Altarian Synthetic would have the unique Altarian techs, but also have the Spark of Life techs too. That way the different Synthetic AIs are actually different.