The twist is what holds a flemish string's loops together. It only weakens the string if you need to make it longer to lower your brace height and have to UNtwist the string so much that there isn't enough twist left in the string to keep the loops from coming unwound. Twisting it up to shorten the string only makes the loop braids tighter and stronger.

The pros of a flemish string are: 1) You have the total number of strands in the string running through the loops. With an endless loop string, you only have half the number of strands forming the loop. So, flemish strings are stronger through the loops. 2) You don't have serving covering the loops on a flemish string, so you can easily see any fraying or weak spots in the loops and change the string before it breaks. I've had way too many endless loop strings unexpectedly break in the loops. 2) The braided loop section of the flemish string is softer than the served section for the loops on an endless string so, for recurve bows, the flemish string is quieter. The braid doesn't slap the recurves like a hard serving does. 3) Flemish strings simply look better. Hey! Aesthetics gotta count for something.

There are no cons for flemish strings, as far as I'm concerned.

The pros for endless strings are: 1) They generally account for slightly better arrow speed than you'd get with a flemish string of the same material. 2) They hold their length slightly better than flemish strings.