Reviving this old thread with my two cents, for what it’s worth. I have climbed l’Alpe d’Huez in Zwift, in BigRingVR and IRL. I use a Wahoo KICKR and always set trainer difficulty to 100%, since, with virtual platforms, 0% is meaningless when climbing as it will make any climb “flat”. When compared to IRL, Zwift is ok, but not reliable and/or comparable in terms of the time it takes and the power numbers. BigRingVR is not perfect, but much closer to IRL on all aspects, gradient, power and of course, the view. I have been using Zwift since the beginning (I was a beta tester) and I no longer use it due to it being way too optimistic with “achievements” (speed, power, distances, etc.) compared to reality. It’s a good social/gaming platform, but cannot be relied upon for serious training. These are my personal observations and your mileage may vary. Cheers! 
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