Just started using Zwift in the past week. What I will say is that on the WC course, I go about 10% faster for the same wattage that I do on a flat road with no wind. And the wattage lines up with what I would expect from HR or perceived effort.
As a comparison, I found myself going 23-24mph on flats while putting out 220 watts - I weigh 166 pounds. This is with no powerups. In real life, I have to put out more like 260-270 watts to steadily go 24 mph, and that’s on my TT bike. Obviously this could be because I am incredibly inefficient outdoors except these numbers aren’t too far off the norms I’ve seen. So my conclusion is that on average, Zwift is overestimating my speed.
All that said, I agree with Stef Levolger that it really doesn’t matter. I am basing my training evaluation on watts and duration, not miles/miles per hour.