We have Compellent systems in the field that have all of the PCIe slots filled and no issues. I think I have seen a cache card fail on a SC8000 once (newly shipped system within 24 hours of operation). 
Cache Card problems on sc8000 controllers Since upgrading from SC40 to sc8000 controllers I have had 2 cache cards replaced. Talking to the field techs this is common , as they are ask to reset the cards first before replacing them. The location of the cache card in slot 7, above the PSU 1 and a SAS card is installed above the cache card in slot 6, does this generate cooling problems affecting the service life of the card?
Cache Controller problems