I think the only problem on a 737 might be the fin losing effectiveness in such conditions. The spinner problem shoud have been sorted by the profile change following the severe engine icing event on a 300 in Germany in the ?90's?.
They are, as exeng says, rare. My only experience was landing a Jet Provost after an hour in icing conditions with huge layers of ice all over it. Mind you, I tend not to look back at the old ships now-a-days as I walk away.....
