I think alot of later failures of Radial engines were due to 2 factors.
Lack of new parts as opposed to overhauled.
Lack of experience in pilots, mechanics and overhaul facilities.
It has become damn hard to find qualified personel in any of the above catagories.
I'd be curious to know what the military records show regarding the 1820 failure rate or replacement rate vs hrs operated. As it relates to engine model and horsepower ratings.
My experience on DC-3's with the pratt vs wright was about equal in terms of reliability. Not too bad at 1200 hp
However on the Super DC-3 with the 1820-80 it was a completely different story.