Yep. Used it, saying out loud Flex are the best, but in practice, if you want to keep the rigth level at each pass, after around 15 passes the quality loss is obvious with default fx, especially filter, degrading sound progressively, ending to a dirty rumble, a lot of dirty low end weird feedback.
Without fx it’s better, but Pickups seemed better anyway with a lot of overdub recording passes.
Maybe I didn’t used the best settings. Prove me I’m wrong! 
My conclusion was, overdub with Rec/Flex, yes why not, and it is a workaround if OT is slaved (overdub with Pickups is not possible if OT is slaved), but wanting to layer many synths like pads /drones with overdub I’d rather use Pickups.
Test it!