While I am new to full time rv living, we've worked on countless class a and c models in the past.
In my opinion, the aftermarket systems, Evans included, in the class a's beat the perceived comfort test/efficiency of any class c Van type chassis we've repaired and road tested. High end truck based class c's however, have very decent factory hvacs.
Assuming all components are properly installed, evacuated and charged, I can only attribute a comparison to available room and/or component sizing and actual heat in the area components are mounted.
There is a lot more room available in a class a chassis and the distance to high heat sources, like engine bay/exhaust typically are greater in a class a application.
I personally have not seen a failure rate one of more than the other down here in Florida in all the years we've been repairing hvac systems.
Like many others have said, no dash air system is going to cool or heat any open interior rv of livable size, that's why many run the generator and house ac's in hot climate driving.
One thing is definite... I'd much rather service a Class A's hvac any day of the week versus the tight confines of a van based Class C. Truck based Class C's fall right in the middle of serviceability.