I've had some of the cheap riser cables in the past (gray with the blue Kapton tape). They have a high failure rate where they solder the wires to the connector. It often is a poor solder. Just the flexing of the cable is enough for them to come undone, so I hate using them.
I've seen some robust ones (like above left) with better materials and strengthening around the cable/connector connection. I only see them for x16, but I need some that are x8 to x8 or x8 to x16. Anyone know of a seller of strong ones like that by in one of these sizes?