There are a few things I would like to say here.
1 CNG is less polutive so it should be used if we are think ourselves responsible citizens rather than egoistics who boast around saying we drive on petrol and you poor users of CNG.
2 There have been multiple instances of people who have driven their cars 200000 kms + on CNG and have never overhauled there engines. Now on average a person saves 1-2 rs per km on CNG so a total saving of 200,000-400,000rs. Now an overhaul at max costs 150,000 if you go to a 3 s workshops otherwise it will be under 100,000rs for the overhaul.
3 CNG does not require a lot of visits to mechanics its our vehmi nature that takes us to workshops every week asking them yar uski gari 150 dati hai meri gari 125 dati hai never considering that they drive in different areas
A final verdict is that if the car has been converted to CNG properly and maintained than its a blessing and BTW who says petrol cars are trouble free if that was the case then there would have been no workshops in the 1990's when the CNG was not introduced. Actually we people love to buy expensive things. Sugar 50 ki hogai hai yar lay lani chahiye, sugar 60 ki ho hagai hai bari mehangi hai daikh kar use karni chahiye, sugar 80 ki hai oh yar 10 kg lay lay allah ka diya sub kuch hai