UNDERsteer, in a rear-engine car? I think you have a bit of confusion in terms there. Oversteer is what occurs when the rear of a vehicle loses traction due to weight imbalance. Additionally, Ralph Nader's criticism of the similarly rear-engined Corvair (and its contemporary Volkswagens) in "Unsafe at Any Speed" had a lot to do with that vehicle's use of a swing-axle transaxle, in which the rear axle's suspension only has one, vertical, degree of freedom and thus has a tendency to bounce upwards during oversteer incidents and risk overturning the whole car. 1969 and later Beetles had independent rear suspension, which does not exhibit this behavior. The Corvair was killed before it could be evolved in this direction.
Additionally, Porsche fanatics will tell you that the 911's rear engine placement is actually an advantage in terms of traction during corner exit, so long as you are not foolish enough to lift the throttle in mid-turn.