I think Paia'aua is quite good on defence, his stats look bad because he is always covering those inside and outside who miss a tackle and he has to attempt the tackle from a poor position.
Yeah, I wouldn't say Duncan himself is a particularly poor defender. He's not the best either, although he had a handful of dominant hits on guys this season.
He's definitely trying to push shit uphill with Kerevi very often on one side of him, with plenty of mistakes on defence from other players as well. He's had some pretty bad misses but they've largely been when a guy has already made a break by beating someone else first.
He's passed the eye test, for me at least. The only thing that's really jumped out (I don't think I've watched any of the games more than once) at me is that he sometimes sits back on his heels when dealing with crash balls in the midfield instead of stepping into the carrier.