I would report my experience with Kettler C12 and Zwift.
I prefer a stationary bike like Kettler C12 instead of my old roller with bike mounted above,
first of all because it’s really silent (while my old Elite roller was terribly noise) and
I can leave my bike in garage ready to be used for a real bike .
I’m using Zwift with Kettler C12 from a week and it works fantastic and yesterday I started my Zwift subscription . It works very well, however note that I’,m using latest firmware Kettler C12 31.27 .
If you have the same bike be sure to update the firmware to latest 31.27 .
I can connect/pair to Power and Cadence and Cardio (bike cardio even if I prefer the external Polar Bluetooth cardio) really instantly , during workout simply I need to change the power using the watt knob in Kettler (and so I can adjust my zwift speed adding more power).
Everything is transmitted correctly to the app, power, rpm, cardio …
There is only one thing that does not work in Zwift with Kettler c12 , but that works fine in Kinomap and Rouvy Android and other minor bike trainers. Kettler C12 cannot pair to the Zwift Controllable power which should apply more power when there is a climb i.e.
I hope to see this feature in Zwift the near future ( if this feature works in another apps I cannot understand why it should not work in Zwift too). However also without this feature the Zwift Kettler C12 workouts are a wonderful experience compared with Kinomap and Rouvy .