I ran LPG vehicles for donkeys years, 120k+ miles but went back to petrol on the next car as the price became almost equal to lpg, yes it ran clean and oil changes were almost pointless as it was crystal clear.
I've recollections of diesel engine having a size before the dpf emissions stuff was required but I'm a KISS guy when it comes to equipment.
Just imagine an LPG running out half way through a job miles from home etc.
I'm guessing a petrol chipper will need to go 3000rpm if it's 1-2 cylinders or 1500 if it's 3-4, other than say clean air zones you lose all the benefits very quickly, it's like a niche within a couple of more niches.