I have icing effect on in my failure settings but doesn’t actually effect the aircraft. I was flying TBM 390 I believe and both engines were ice, both leading edges and on the vertical stabilizer but aircraft had no issue with maintaining speed or altitude. In real life that would be a BAD day for a pilot.
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I’ve noticed this. I suspect that the amount of icing you can see is much more than is actually on the aircraft. I also noted that the icing effect was very aggressive.
Of course, this is a “nice to have” feature and probably will need some tender love to work fully and correctly.
I was flying in the tropics (East Timor) and picked up ice at 9000 ft (!) … in the tropics. Structural ice incl. windshield ice would not disappear until I descended to below 6000 ft.
In the Cup, I did notice that more engine power was required with ice.
Just today, I flew into a thunderstorm over Cebu / Philippines (again: tropics). Real ground temps never go below 24°C here. Current temp is 28°C. In the FS2020 predefined “storm” weather, we had 14°C at ground level (sea level). Flew around in a C172, picked up ice. It only melted off at around 13°C OAT. Did not recognize any ice-related decay of performance or flight characteristics, but then again, I never flew a C172 into icing conditions in real life 
It seems that cabin heat, sublimation etc. to melt / dissolve ice are not yet correctly modeled.
All in all, I hope that Asobo will continue work on this icing effect / feature, for it certainly adds a handicap to the sim pilot and realism to the simulator.