I bought my 2004 model F 250 7.3, 2nd hand in Dec 2006, with 104,000 kms or about 60,000 miles on it.
A few months later in early 2007 I had our equivalent of what you call "CNG conversion" done (We call it LPG, Liquefied Petroleum Gas but really it is a mix of whats known here collectively as "autogas" which is Butane and Propane mixed).
This is the dyno figures straight after fitting.
Bear in mind this is stock standard 7.3 with no performance modifications i.e. DP Tuner chip or enlarged dump pipe exhaust etc...
Now in 2012 some 6 years and another 40,000 kms (24,000 miles) later the LPG side of it has given me no trouble at all.
That said - I DID have an unrelated issue with a Injector Drive Module located up inside the left fender and all 3 specialist diesel mechanics etc who had a 8 month long stint at trying to diagnose and fix the problem, blamed the LPG conversion, about which non of them knew diddly squat!
I guess we have very few F 250s here and a lot of remote places with no Ford dealer - so being mining industry areas most of the diesel fitter / mechanics are all used to working on Cat diesels from mining equipment, and the F 250 with its Navistar T 444 E and throw in a LPG conversion and it all gets just "too hard" for them!
Had I NOT bought a Auto Enginuity brand Diagnostic tool and software from the USA and had it air freighted downunder, I reckon they would still be scratching their heads and trying to figure why one bank of 4 cylinders just stopped working!
It was my own fault - I drove thru a brown water muddy puddle up to about wheel hub height on the left passenger side - and the mud got over the electrical connectors on the IDM module up in the left front wing panel - and because the local area was apparently full of the mineral "magnetite", it stuck to the electrical pins on the IDM and created an electrical short that no one seemed to be able to clean off, to make it work again.
$1500 and a new IDM module controller from the USA, and it worked again and hasn't missed a beat since, but I am very leery now days of any water crossings.
All up that repair was 8 months off the road and over $8000 in total to repair by 3 different repairers (Which Ford Aust wouldn't cover under my useless $2000 extended warranty).
At the time I didn't have a choice about the water crossing - there was a cat 4 cyclone bearing down upon my somewhat hairy clacker, and the only way to get the boat and my family outta there post haste, was to drive thru this small stream across the road from the boat ramp, that the pre-frontal rain from the cyclone had created.
It wasn't until a few days later when that muddy water with the magnetite grains in it, dried inside the rubber electrical connectors shield on the IDM controller module that everything went to crap..
Anyway the point I guess was that every mechanic who can't diagnose whats wrong with your diesel will blame the LPG conversion - its an easy excuse for them...rather than admit they don't know shyte from shinola when it comes to diesels that have computer controlled electronically fired injectors!.
Least that's been my experience for what it is worth. The Gas is a good system and works well - but mechanics will just shrug their shoulders and say the LPG / CNG is the issue - when they can't figure whats wrong with your engine.
To most mechanics it seems the gas conversion that runs in conjunction with the diesel is some kinda "white mans magic" best avoided at all costs.
All 3 reckoned if they removed the LPG they could have fixed my engine - when it was nothing to do with the LPG, but they all wanted to rip it out!