A better reason for the Romulans to go "ham" on synthetic life forms would be strategic. If you hate the Federation, but can't do anything to counter the Federation at the moment, due to your homeworld's sun going "supernova", you would create a crisis that would ensure that the Federation did not have a strategic advantage over you. Thus, you would hack into the synth's network on Mars to destroy a major Federation military facility: Utopia Planetia. The secondary objective would be to squash the manufacturing of synths, thinking that all the Starfleet needs to do is to request a military-grade synth, and produce it by the thousands. Think about it: the Federation could churn out an army of "Data(s)" in short order. That would scare anyone, especially the Romulans, who are in no position at the moment to counter a potential threat. Thus, playing the long game, you make sure that all synths are destroyed, and that no new Synth research is conducted. This, IMO, is a better reason for the anti-Synth plot, rather than some prophecy about the coming of "Space Mecha-Cthulu" or whatever.