No matter how well built the riser is, it can still cause major upset for signals and data timing. The PCI-E bus isnt designed for extra length with more connections and dissimilar metals in the chain, especially running at full speed. Each slight change in impedance (from connections and changes in metal) cause part of the signal to be reflected back where it came from, increasing noise, decreasing signal strength and further changing the time a signal is accepted as valid (due to taking longer to reach a valid level) as well as the extra propagation time taken to travel the length of the cable. Thats not to mention the extra interference to signals and data when they are placed in the open with nothing to protect them. A PC is noisy! If you have a problem when using one, make it a prime suspect.