LPG converted is ok if everything is set correctly...the converted cars that have cylinderhead problems is due to improper tuning....meaning to say it has been set too lean...so you may think corrosion is in the head when in fact the valves and head are being damaged due to and overlean condition...
Most of us expect that if our cars are LPG powered it can run more economical in terms of kilometers/liter when in fact a properly tuned LPG powered engine should have less milage than the gasoline fed engines, you just get savings from the lower cost of the LPG. EXAMPLE: If your car gets 8km./liter on gasoline, when converted to LPG it must only get 6-7 km./liter of LPG..then it is properly tuned...and if you compute....LPG power will still come out cheaper. But if you will be getting higher milage with the LPG then definitely it is very lean and you engine will have premature damage. A properly tuned LPG car will have sparkplugs that will last 5 time longer that gasoline fed engines...and oil change will be sell frequent. If you have a lpg powered car never run on gasoline take a look at the sparkplug and its still vey clean after months of running.
In my opinion the only bad thing about the LPG conversion kits are the quality of the components. I have a HIACE that has been running on LPG for 2years...no problem at all with the engine, the the LPG is very problematic after 2 years...the selonoids alway get stuck, the diaphram gets dirty and dili na mo start....so hassle jud, but if everything is working ok pud kaayo. The only problem is you dont know when mo kalit ug ka guba, maybe bad quality and selonoids...so I just stopped using it cause after the 2 years I used the kit murag daghan na problems ang LPG kit, unpredictable na...but the engine wala jud problem at all.