Online research has given me the impression that commercial riser cards (even the expensive ones) are plug-and-pray devices more likely to fail than not, but I find it hard to believe that a product so common and useful could be so unreliable. Is my perspective skewed, or are risers really so chancy that it's a surprise when they actually work?

I ask because I'd very much like to reposition a classic-PCI, pro-audio interface and a PCIe 3.0 video card like the HD7970, preferably by mounting them on cheap, flex-cable risers like this with added EMI shielding and Molex/SATA power connectors. Is this a viable course of action? If not, any suggestions?

As long as you're not using a Sandy Bridge or newer platform, I don't see why you would have any problems with using a PCI Audio card. Many companies are being very "hush hush" about the issues with PCI cards on the Sandy Bridge platform, but it is very visible in M-Audio interfaces, and even PreSonus has mentioned the issues on their website. ADK, a company which builds DAW machines has a lot of absolutely invaluable information about the problem, and one of their techs has gone as far as to write that the issue is apparent with ALL PCI interfaces, no company is immune.

Some users find that low end, consumer sound cards will work fine, but because they do not have the same requirements or abilities, they do not hit resource limits or problems that most professional cards display in very short periods of time.

That's very good to know and I'll definitely keep that in mind the next time I upgrade, but what I'm currently concerned about is the effect of the risers. Sorry for not making that clear in my first post. I've rewritten it to fix that.

I guess I missed a few words in there. I don't see why you'd have any issues with the risers. Whether it's a ribbon cable or 90* adapter, I've used many, and never had issues, I don't see why you would.

Your point of failure from my perspective is interference, but I don't think you'll have any problems. Of course I could be wrong, but the funny thing about the internet is that you can read the 100 times a product failed, and completely miss out on the thousands of products that are working perfectly.

I actually had some problems with the flexible riser I used on my GTX 660ti. At first I had the Delock riser card pcie x16 but this one was very unstable and I had problems booting. I then ordered this http://www.ebay.com/itm/PCIe-x16-Flexible-Extender-Cable-w-Molex-/150620070281 (I bought mine from Cablesaurus). At first I also had problems with this one, but when I downgraded the BIOS from Pcie gen3.0 to Pcie gen2.0 it works fine (I have the Ivy Bridge i7 3770k that supports Pcie gen3.0). According to these reviews the differens between Pcie gen3 and gen2 is not visible:
Techpowerup: http://www.techpowerup.com/reviews/Intel/Ivy_Bridge_PCI-Express_Scaling/23.html
Hardocp: http://www.hardocp.com/article/2012/07/18/pci_express_20_vs_30_gpu_gaming_performance_review

I have not yet found a riser card that supports full bandwith for Pci express gen3. Some say that AMD graphic cards are easier with risers than Nvidia cards. If someone find a riser that support full Pcie gen3 I would also be interested, or if someone have another solution. Not because of the performance, but then I wouldn't have to adjust the BIOS (not that this is much work either, it's just when the BIOS need to be reset and dosn't affect the daily use).

Thank you very much for the advice, guys. Sorry I didn't respond earlier, but I've been swamped.

So it sounds like PCIe 2.0 works fine, but 3.0 is a bust. I'm not particularly surprised, but I am disappointed. Sounds like I better work out a way to keep my GPU off a riser.