In fact, there are already cases where initially written by h2pas wrapper is totally rewritten by hand.
That's mostly because such a user has no clue where its for and how it works.
Rule number one is to first run a c preprossesor to expand any C macro's to actual plain C and only after that run h2pas and h2paspp.
Running the C preprocessor means also pulling in all C header files except maybe the standard ones.
After that I almost always have excellent results with h2pas.
Notable crime that doesn't work: K&R style parameters with type only, like (int,int,long void*) // valid C but....
AAlso note all but some wince standard headers are already translated, of course.