Poly thread is much stronger then cotton. Remember the myth that poly stitches will cut through cotton fabric if used for piecing.
I rarely use 50wt 3 ply for piecing. I use 50wt 2 ply with 60wt or 70wt in the bobbin. 50wt 3 ply feels like rope to me. I have Aurifil 80wt bobbin thread I use a lot. My bobbin seems to never run out. Those that melt poly thread must use a red hot iron. I've never had poly melt by pressing a seam. I would suggest watch the Superior videos on you tube to understand perfect tension. Star with 0 and go up until the tension is right. Be sure you have the right needle size too.