The extended V-cache, at least in these early releases for the tech is apparently much more susceptible to issues caused by voltage variance and heat, which is why voltage has been locked down. Apparently one of the reasons they perform better in many cases when undervolted, as the reduced heat allows the core to run at higher clock speeds, without breaking the voltage/heat tolerances specified for the cache. From everything I've read, I don't think its that they were fragile per se, more that they are just more susceptible, and AMD wasn't willing to put out a part with a higher risk of failure, given it was targetted at enthusiasts who are more likely to tweak and abuse the processors, thus increasing RMA rate.
I suspect in future and as the vcache tech matures, they may take the foot off this brake, but wasnt something they were willing to do on intial release (as increased RMA rate would give the chips and therefore the technology a bad reputation; definately not something they wanted).