Nylon thread melts at a lower temperature than poly or cotton, goes brittle with age, and doesn't hold its color well. It's sometimes used in quilting as a monofilament thread, but even then poly monofilament is a better choice. It can be wiry, hard to handle (doesn't want to stay on the spool), and comes undone at the end of your line of stitches. My only experience with it is the monofilament thread, so there may be other types that aren't as bad. It would definitely not be my choice for piecing.