Flex ducting in addition to putting a U in the ducts puts more
resistance on the flow being discharged from the fan which impedes its
performance. In addition, any "trapped" water in that U would also put
resistance on the flow.Putting in rigid duct with minimum bends would
decrease the chances of the ducts sweating since it will exhaust
better. In my opinion, if you leave the fan on for even 5 minutes after
a shower it would be enough time for the fan to clear the bathroom of
humid air and by the time the fan is turned off, the likely hood of
moisture in the duct will have greatly decreased.