just make sure when you burn that string end you smash it good and flat on a hard surface.
Doing this actually makes the melted end weaker and more succeptable to pull thru than if you fluff the ends and burn a ball on the end.
To get a good ball you first have to use the right loop material. It needs to be made 100% of either polyester or nylon
including the inner fill. Too many use some sort of junk plastic like fiber fill that is crap for strength and melting a good ball.
Zeneth and Brownell make good loop material.
Oh yeah, I forgot aout the original question. [8D]
Been using a loop for 13 years now and not once have I had one pull thru or break. Use the right material and tie it correctly and you will have no problems at all.