It has occurred to me that direct drive trainers may be a big advantage in zwift, as they measure watts at the cassette/axle whereas wheel-on trainers measure it at the tire. Thus the rolling resistance of the tire must be factored in. For the standard trainer tire this is probably 25-30 watts [less if using a higher end racing tire, but who would waste one on a trainer…].
So direct drive cyclists have this wattage advantage, no?
I would add to this, that wheel/ tire inertia must also be overcome by wheel-on trainers, for any acceleration/variation in speed. This requires extra effort, and is independent of the rolling resistance issue.
So it would seem that even with accurately calibrated trainers, there are significant differences related to the design of the trainers that affect the power and performance that a given rider produces on zwift.