There is nothing quite like driving in the snow in a front-drive Cadillac with old tires. Rather than mere "understeer", the problem could better be described as "no-steer". It's a strangely amusing sensation to be gliding along at 30 or 40 MPH and turning the wheel from stop to stop with the car not making the slightest change in direction. Oddly enough, that same car with good tires on it drove as if it was a snowmobile.
The rear-drive Buick performs on snow about as one would expect a 2-ton vehicle using mostly a 1970's-era suspenstion design to perform in that kind of situation. It doesn't get stuck unless you drive like an idiot, but the rear end tends to be a little squirrely. My wife refuses to drive it. Her Grand Marquis isn't really any better--it doesn't oversteer as much, but the drive wheels seem to have a lot less traction to actually get the car moving.
J_A_B