The OSSC can take all of composite video, luma, and clean composite sync as a composite sync signal and shouldn’t have the screen centering issues.
I still think it’s weird that only these consoles are having issues, when the Dreamcast is ostensibly also using clean composite sync, unless you have an aftermarket SCART cable that’s, weirdly, wired for composite or luma.
Do you know if you have straight-through SCART cables for your Genesis, Saturn, and N64? I know the Genesis and Saturn output TTL (5Vp-p) sync, but your TV is expecting 75Ohm (1Vp-p), so the SCART cables for these systems need (per RetroRGB) a 470 ohm resistor and 220uF capacitor on the sync line, meaning a cable without components will be insufficient. The N64 RGB mods are capable of TTL by bridging solder on a jumper. (PAL SNES doesn’t output clean composite sync, so that SCART cable is probably wired for either luma or composite video.)