if u get an original cng kit nd QC passed clinder.theres nothing to worry.believe me.but ppl,just to save sum money,put the cylinders which are used in household cooking in the cars' trunk for using as cng containers.those are deadly.very dangerous.f u dnt do this,cng isnt harmful.the cylinder is very very hard nd wont blast of until hit ny sum really really massive thing like a high speed truck or a train frm the back.but yeah,1 thing is ossible that the cng leaks in accident nd can cause fire,but petrol can too...!!!
the 2nd thing is,engine life.cng DOES NOT hav bad effects on the engine of a car until u maintain it properly.3 things are needed.
1) regular oil changes(more frequently then use of petrol)
2)frequent cleaning of airfilter(lets say 2k kms nd changing it every 10k kms)
3) changing plugs every 15k kms at max.
u keep these things going nice,it wont do anything to the engine.trust me...