Both the Vario and Steermatic cars are CVT. Both hate CNG as CVT relies on the throttle sensor inputs which are affected by the flow of cng.
My understanding at least of why CNG is bad is based on the fact that petrol is delivered at a continuous rate by the petrol pump thus the throttle sensor experiences no lag or jerks which confounds the CVT computer.
CNG flow, in pakistan at least, is not as consistent as petrol due to air/oil in cylinder etc which affects the throttle sensor which results in jerks/interruptions in the signal sent to the CVT which causes it to skip a beat every now and than which results in extra strain on the transmission. Also CVT fluid needs to be changed at regular intervals (as all AT cars require) which if neglected causes failure in the transmission.
These are just my opinions on why CNG is hard on CVT's. If anybody thinks i am incorrect, feel free to correct me