Apart from the fragility of the worst of the knock-offs (which are likely to carry name-brand markings at a way-too-good-to-be-a-white-lie prices), my worry would be adequate shielding and twisting together of the appropriate wire pairs/triplets. That would show up as a smearing of the signals (you'd need an oscilloscope to actually see it).

In functional terms, it could affect the WWT* signal on very short duration flashes (for really close work, near the flash's minimum rated range) and possibly the chatty control signals that go on during high-speed sync. At close ranges it could significantly overexpose; at longer ranges the extra microseconds won't make nearly as much of a difference. You can test HSS fairly easily -- if it doesn't work at high shutter speeds, you may still have a perfectly adequate cable for X-sync or longer.

If the cable is at all well-made, it should have no problem, but if it's more than a couple of feet long and is just a bunch of straight wires in a jacket, it might show high-speed signalling problems. If things are really, really bad, your neighbor's garage door might open every time you take a picture -- and if that happens, it's worth hanging onto the cable just for that.

*WWT is a TLA for the French Canadian exclamation, "whoa, whoa tabernac!" It's a common name for the shut-off signal in any piece of Canadian military electronics -- talking about the "WWT strobe voltage transient" is much more impressive to passers-by than talking about the off switch signal.