Flex is so you can play with your friends or guild (any number besides 10 or 25), so you DON"T NEED TO PUG.
If you choose to pug, it will be unbearable.
Flex is scaled from normal. Would you take 9 random idiots into normal? No. Then why would you take 13 random idiots into flex which theoretically is the same difficulty as normal?
Get your paper dictionary out and look up scaled, obviously google and wikipedia failed you.
As for the thread title: Flex worse than LFR? That awkward moment when....you find out your friends suck at wow?