Butcher and Thompson for Ristolainen is a very good trade for both teams. I think Ristolainen is the classic "change of scenery" guy -- Buffalo deployed him as a "shut down" guy, which is not his best role. He's far better defensively than the analytics crowd suggests and offers nice physicality, but he's really more of an offensive D who hits than a defensive-minded guy. Also, I think it's important to note that Buffalo is just a mess right now -- almost the entire team underperformed for two whole seasons -- they are poorly managed, poorly coached and the prospects are poorly developed. Any player leaving there right now should see a bump in the quality of their play.
Ristolainen is an interesting case scenario in that no one seems to understand him. The same people who claim he's one of the worst defensemen in the league are the same people who say Will Butcher is one of the best two D on the Devils, so it's safe to say we can discount their voices. Buffalo never deployed him correctly, I think most people would agree with that. He's a big kid who can skate and shoot very well. He's got a nice compete level, which makes up a bit for an average hockey IQ. He'll make some mistakes, but he'll also make some plays. I don't suggest bidding high for him, but the talked about offer of Butcher and Thompson is extremely reasonable for a player who could score around 40 points and add some much-needed physicality to the Devils back 6.