I know that the turbine used in the V-1710-127 turbocompound (basically a turbine from the C-series turbochargers used in the P-47) could not cope with the temperatures of teh V-1710 at full WEP, even with copious amounts of ADI. Allison wanted to develop an air cooled turbine, but without customers for the engine they did not want to fund is development.
I wonder if the problem with the R-3350 PRTs was the turbines themselves, or the drivetrain used to feed power back to the crank - which included fluid couplings/torque converters.