So my brother is an engineer, and he said that the loops in an endless loop string aren't necessarily weaker than the main length of the string. We even did a test with string where we broke several strings that weren't served and they broke in other places and not the loop. That really surprised me. as soon as we closed the loop with serving though it would break where the angle changed. So yes the loop is a weak spot but not because it appears to have half the material, but because closing the loop with serving causes an angle change with changes the forces. Therefore if you make your endless loop based on the main section strength and not the loop strength then you shouldn't have any extra mass, you would actually reduce mass. Therefore I think I've come to the conclusion that endless loop would be the best method if you're going for optimal performance as long as you keep it waxed and avoid snags. But depending on your purpose Flemish twist or other methods would do perfectly fine, especially if you are using an extremely stiff material like Dyneema. Also, Flemish twist will always have its place in natural plant fibers since you can't splice in threads of shorter length to make a longer string with an endless loop.
