in my opinion wheter it is FFitted /outsiders fitted , it depends on settings how well they install
i have personally installed cng in various cars from outside not FF & also drive lots of F/F cng infact drove XLI/GLI with F/F cng too, no difference if it i sproperly installed
actually unfortunately on every streets who can fit cng kit claims to be cng expert but infact they do mess with cng stuffs ,due to which cng gives problem
due to such non technical mecahnics, people think that cng is not good, cng is fault bla bla bla
please try to understand it is just fuel type, not rocket science only required good installation
even before 20 years, before use of cng in pakistan, what there was no problem in petrol cars at that time? any bad mantainance , poorly installation etc. etc. caus eproblem,
so not to blaim for cng itself, it is fault of those who misconfigure it.