I had a '79 Impala full-size wagon for 11 years. It had a huge engine and a positraction rear end. Without a load in it, it was lost on slippery roads. But loaded, and with old-fashioned snow tires on the back end, it was amazing. (I added air shocks to cure the bumper-dragging problem.) When needing to start from a stop on a steep uphill, the weight shifted onto the rear drive wheels. No problem. Fwd was the opposite, shifting weight off the driving wheels. Despite all the pro-fwd bs around at the time, the wagon could get up snowed-in logging roads far better than any fwd. Throw a set of truck chains on the rear wheels, and it was as good as a 4wd without chains.
A problem with fwd on slippery roads is if you go into a corner too hot, and suddenly lift off the gas, you'll lose the rear end and fishtail into trouble. Rwd, on the other hand, is easy to hang out the back end and countersteer around slippery corners. The Impala could do this in ho-hum simplicity.
I also had a small fwd sedan. It was completely hopeless in slippery conditions. The heavier it was loaded or the steeper the climb, the worse it got.