As Acetate said, they foul up the grounding, unless they're made of twisted pairs (TP). TP is so much better that it should be reliable even out to 3 feet (I've tried my homemade 5' flat TP cable to ATA100).
Also avoid rounded cables that have metal braid on them because they too can worsen the signals, and most don't even have the braid grounded.
Also avoid rounded cables that have metal braid on them because they too can worsen the signals, and most don't even have the braid grounded.