In my early days of glass work I used polyester because of the cost. I am prepared to accept that in those days some/most of what I did was questionable. I was still learning.

Two of my early polyester molds twisted after a few weeks of sitting. I found out that polyester does have a tendency to shrink and in my case twist as it cures. I have never had that problem with epoxy resin molds or parts. I have never used polyester since then.

Ed S