It's how tightly woven/dense it is--not the fact that is a sheet. I have bought some duvet covers that are hard to sew on--and one I had, while beautiful to look at, was a horror to work with when FMQ. My needle kept skipping. I finally figured out it was because I bought this gorgeous fabric (in the form of a duvet cover) that was extremely tightly woven. I ended up using it anyway. I think I changed to a smaller, new needle. I don't really remember.
But anyway, I use sheets often--and almost always for the charity quilts I make. I look for all cotton sheets, but for the charity ones I may use one in very good condition that is 50/50. Why not? Its original purpose was for bedding, after all! Seems appropriate to use it for a quilting/blanket which is also bedding.