Very Sturdy. I’ve used foam before, and a cork + foam together in some cases (desperate times, ok?), and my practice would always suffer when using foam. There would be no stability, I struggled with grip, and in some cases, the foam block would collapse. Never experienced those issues with a cork block, and it doesn’t become a distraction.
However! It is good to note is that for head or back support, the firmness of the cork block may be quite uncomfortable. You may find foam is better here.