bhai it's a long discussion but keeping it short.
first note that i HATE cng honestly hate it...there's no throttle response on cng.
coming to the point bhai cng can't harm the engines life means the main thing is Blast.....whether it's produced by petrol or by cng it's blast......in carby engines carb's are effected by cng as there's no lubrication in carb because they are running dry,so usually carb get's damage by long term usage of cng....in efi i'm not sure that injectors or fuel rail gets damage but i think it does.....if anyone disagree from me that cng effect pistons(rings) or valve seals (which usually decreases engine life) then bring up with a logic.
But Bhai don't install cng.