"I wonder if the stainless steel Harpy will be more durable for heavy work,..."
Technically, yes, SS will be more durable as far as brute force is considered. Then again, whatever you are doing to break or weaken the FRN handle of a knife, you're probably not using it appropriately as a fine cutting tool--but as something else like a blunt instrument.
"...or is it just the internal mechanisms that count?"
Yes, if you are using your knife appropriately (no prying, chopping, hammer pounding, etc.). FRN in Spyderco form has the advantage of being grippy and extremely lightweight while it is also extremely durable and impervious to adverse elements. I can testify to that. HTH
P.S.
Welcome to the Harpy Club.
I recently acquired the Harpy with G10 scales, a discontinued model unfortunately. With this one I found the perfect answer to the FRN vs SS dilemma. G10 has the 'grippiness' of FRN and the similar heft/solidity of stainless steel, IMO.
Check one out and you'll see.