Chassis flex is like adding a secondary soft progressive spring to the shocks, because the chassis tweists before the springs on the shocks react. (but it can make the car react lazy, on a big flowing track its not a big deal, on a tight technical one it is) A flexi chassis works better on a rough track I know that from Kart racing. But that said it depends on what frequency the chassis flexes at and the first order harmonics of the chassis. A stiff chassis is harder to tune but once its set it stays that way.
A flexi chassis will add steering, usually more than any gain in rear bite.