I’ve used the workout mode a couple of times (a couple of FTP tests and a custom workout) and have noticed that the resistance does not behave as described in FAQ’s etc.
There seems to be 2 issues:
When ERG mode is disabled the resistance varies subtly with gradient. I thought this was psychological but on my 3rd workout (today) confirmed it’s not me 
Essentially the resistance falls away slightly on the descents so at constant RPM watts fall from say 330w to 295w. I have an activity on Strava that shows this quite clearly. As soon as the descent is over I find that I can get back towards my target of 330w. My understanding is that there should be no change in resistance regardless of the terrain… which is what you want/need.
When ERG mode in enabled my cadence/rpm seems to be driven downwards. I have to really fight to keep it in the 80-85 range when I really want to be doing 90-95. When I start to struggle and momentarily fall below the target power my cadence ends up falling… so that by the end of the section I’m pedalling at 70 rpm even though I’ve essentially maintained the required power. My expectation is that I should be able to pedal at 90 rpm (say) and the trainer should adjust the resistance accordingly.
I’m not sure if the above represents a problem with Zwift, the Tacx Neo trainer or a combination of the two, but it would be interesting to know if others are seeing these issues.