You know how people always tell you to test the strength of the thread by tugging on it between your hands to see if it snaps, and that'll tell you if it's "good" or not? I hear this most when people talk about having purchased thread at a garage or estate sale. I can snap my Aurifil 50/2 very easily with this test. I can't actually break any of my serger thread. I quilted with serger thread before I knew I wasn't supposed to. I've never had any of those stitches break and all of those quilts are still going strong (I wash in washing machine on gentle). On my daughter's bed quilt, which I quilted with Aurifil, every time I make her bed and tuck the quilt between the foot of her bed and footboard, I hear stitches snap. I'm going to have to check it one of these days and try to figure out where it's snapping.
Ymmv, but that's just my experience.