With the retrotink I’m getting a lot of noise in the image, especially on the PS1, Dreamcast and Gamecube.
I’m not certain what the problem is, but visual noise tends to be caused by poorly-shielded/unshielded video cables where composite video is used as sync, or by dirty power sources, which can be cleaned up by using first-party or known-good PSUs (possibly also some form of power conditioner or UPS if the power in your house is really bad).
You might not be seeing this interference or much of it with the generic scaler or your TV, because they may have additional video processing or stronger low-pass filtering for analogue sources that the RT2X-SCART lacks.
The N64 doesn’t even work with it, the image is mostly black with a very faint ghost of it. I watched reviews and they said that the retrotink works with the composite/s-video and I saw people using it with the Nintendo 64.
The regular RT2X and the RT2X Pro support composite video and S-Video (as well as 15kHz YPbPr); however, the RT2X-SCART is a different device, which only advertises support for RGBS input. If your N64 hasn’t been modded to output RGB, then this would explain the black screen you’re getting.