I use serger thread all the time for piecing quilts. Also, I use it in my longarm for quilting and have not had a problem with it being "weak". It is a thin thread but by no means a weak thread. Most serger thread is poly.

I am saying it may be too thin for denim because of the weight of the fabric. But you might use a serger for the denim quilts. It makes a perfect quarter inch seam.