Again, the limitation is not with SCART–it’s up to the display to be compatible with the video modes output by these consoles (Or you normalize the signal with framerate conversion on a full scaler, like a Framemeister or the upcoming OSSC Pro).
Older consoles like the Genesis, SNES, and Saturn output 240p or 480i at somewhere around 60Hz (Or 288p/576i@50Hz for PAL variants), which are easily line-doubled by the OSSC to 480p; however, the framerates usually enough off-spec from the normally-supported 60Hz±1% (Including 59.94Hz) that digital, flat-panel display won’t accept the signal (I don’t think the Dreamcast really has these problems). CRTs, the displays these consoles were designed to use, don’t really care if the input is that off-spec.
Additionally, the SNES (and the NES) has a janky sync signal (short version is that the scanline right before the first visible scanline on every other frame gets cut short, which results in an uneven sync), which a lot of modern displays really don’t like, and which is fixable only with a scaler that can accept the signal or with a dejitter mod.
If you want to learn more about this sort of thing, check out My Life in Gaming’s RGB Video Master Class series on YouTube.