Some time ago whilst browsing the site here I ran into a thread/post where an EC had a coil of 12/2 NM melt down when he used it as an extension cord. If I find it I'll edit this with the link....
I think with the right conditions of coil wrap it is possible for a choke or inductor-like effect to cause excessive heating, with enough build-up to cause damage. As reno pointed out, there are many cord reels in use and statistically, how many have failed in this way? Probably not a lot.
And as rescapt19 mentioned, I'm sure a metal core/rod would act to increase heating effects.
