I'd seriously reconsider it as a medium by a few GHz. Full shielding would probably be needed below that point.
SATA cables are basically full-shielded ribbon cables made of better-performing materials.
As for range vs. frequency, given reasonable drivers and receivers -- PVC insulation isn't fantastic, so don't expect any miracles. At 12MHz, and average logic type transceivers, it's probably 10s of meters, maybe a bit over 100. Further than you would ever want to run a ribbon cable (for a number of reasons)...

12MHz, that's comparable to the bandwidth of old school hard drives -- cool fact, the ST506 interface used single-ended (TTL level) control signals, chained between one or two drives and terminated with a resistor pack in the control board and in the last drive; and differential (RS-422 level) data signals (from/to the head amplifier, point-to-point -- no split cable allowed), terminated with 120 ohms or so. It was high tech in 1980.

I would be more concerned about the EMI implications of the arrangement -- if you're doing a one-off and don't have any particularly sensitive equipment nearby, you probably won't mind the emissions, but the susceptibility may still be an issue. At the very least, assign every other wire to ground, and connect them to ground at both ends. This gives a typical about 100 ohm impedance for the each signal line inbetween, so yeah a little source termination resistance is desirable (most logic devices have a 50-100 ohm source resistance, so a little extra to make up the difference is about right, or more can be used to slow the risetime further).
50cm is short. Short enough that inter-integrated circuit (I2C, I2S, SPI, etc.) signals are very possible, even in a full commercial setting, without a shielded enclosure, testing to EMI standards. In addition to interleaved grounds, it might also need some ferrite beads, and cautious filtering at the transmitter and/or receiver. RS-422 signalling would be wise, but may not be required. (I would strongly recommend RS-422 and/or shielding for anything much longer.)
Tim