from my point of view no disadvantages at all ..
With increasing price of fuel, its better to convert your cars on CNG.
The only temporary disadvantage in my view is that your warranty will be void if you install CNG in your XLI. So if you dont wanna loose it, drive it on petrol till then and afterwards convert it to CNG.
Advantages- Cheap alternative fuel, will be easy to sell if petrol goes even higher, which is highly likely since the price of crude oil has reached 113$ per barrel in international market.
Disadvantages- one one, your car warranty will become void.. Thats all... None other in my view.