DC Rainmaker did a Neo 2 in-depth review, which includes a power accuracy check using TR. There is some variation around the set point, but in general you can see taht the trainer is very responsive and quite good at maintaining the power target. The fluctuations are always larger on higher power intervals since we all tend to be not quite as good at maintaining a set cadence, so the trainer hunts around to follow our variations. The key point, particularly for those who are less used to the peculiarities of erg-mode training, is to forget about power and focus on cadence. Trying to adjust the power yourself by pedaling faster/slower will only increase the fluctuations.
Did you try erg mode with other apps?