I've been using serger thread to piece with my computerized Pfaffs forever, and I've had NO trouble. Before I bought my first Pfaff in '99, I used a computerized Viking for 12 years with serger thread. No trouble with it either. The thread has held up in all the quilts I've made. I usually take my machines to the dealer about once a year for a 'tune-up', and I've never been told there was 'stress on the tension disks.' It usually just needs cleaning in the places I can't reach.
The reason you use 3-5 threads in a serger is to overlock the edges of the fabric and prevent raveling in addition to sewing a seam. The thread is thinner so it won't wind up making a heavy seam that would show through on the front of the garment. Polyester is a VERY strong thread.
Right now, I'm repairing a quilt that one of my grandsons has 'loved' a lot. The thread didn't break - it wore out where he or the dog chewed on it, where there was too much stress in places where I didn't do enough hand quilting, and on the edges of the binding.
The quilt police haven't raided my house yet for using serger thread in a quilt! LOL