Sync on green standard was cheap and easy. It was usually adopted for low end video cards and monitors.

Most high performance video cards for the Mac II onwards used combined sync or separate sync. I have never experienced problems using modern TFTs with an analogue input with SuperMac and Radius video cards. Possibly you are unlucky with your choice of monitors, but if the TFT is cabable of ~60Hz it should work. The Radius Precision family work for me.