From the view point of a sewing machine mechanic, we can't get a balanced stitch with Coats & Clark. But that doesn't really mean anything to anybody else. If you're balancing the tensions with a zig-zag stitch, you can't get the "tick" on the back even.
You'll notice that Coats & Clark doesn't make a cotton wrapped poly anymore. It's 100% cotton or 100% polyester.
I have two wedding quilts from 1984 that the polyester thread cut through at the seams. So, from that experience, I only use cotton thread for piecing.
However, I use polyester thread for machine quilting. It behaves so much better than cotton thread.