My last employer forbade departures in freezing rain although landing was allowed subject to braking action.
The problems are that freezing rain adheres to all surfaces around zero and below.
It is extremely difficult to deice except with heated fluid.
On untreated surfaces the braking action is a gnats f*rt above zero - a neighbour landed a DC8 and had to use differential reverse thrust to steer the beast.
On approach I haven't found it a problem but due to anti icing only available on leading edges I wouldn't consider departing.
It is difficult to see the deposit on the wings from inside the aircraft - as dead heading crew I was sent forward to the cockpit as even the ground crew who had inspected the wings hadn't noticed the glazing. (my skipper was seated adjacent to the over wing exits).