Dude, I installed it when mileage was 8000km and now the mileage is 32000km and Alhamdulillah I didn't face any single issue so far No RPM fluctuation which is pretty common in CNGed vehicles,
The good thing I followed since first day is that I used to fill the gas from same CNG station (Mostly) and apart from that I just did the same routine check ups after every 5000km.
Pick is 95% same as petrol and the plus point is no knocking on CNG.