Nunn did well to get that opportunity in Miami. Right place, right moment.
His solid play there was helped greatly by playing next to Jimmy Butler (a playmaking wing) and Bam Adebayo (a high post facilitator). Two players who the offense ran through rather than through their PG.
Nunn is not capable of running an offense. He can only operate next to other players who can take those duties off his hands. He is a solid defender and provides some scoring but his lack of outside shooting makes him a dodgy fit as an off-ball player next to those playmaking wings (like LeBron). That is why he will only ever be a short term fit next to them before they move him on.
Nunn also benefited from having a lot of shooters around him in Duncan Robinson, Tyler Herro, Goran Dragic, Kelly Olynyk, Meyers Leonard, Jae Crowder. They helped compensate / hide Nunn's dodgy outside shot.
Nunn would not have started on pretty much every other team in the league while starting in Miami. It was a near perfect storm for him (great coaching, playmaking wing, playmaking big, lots of shooters). It will be very difficult for him to replicate that elsewhere; to get a comparable situation elsewhere. That is why I believe it will be the high-point of his career unless he manages to improve that outside shooting.
I believe Nunn is closer to fighting for his NBA future than he is to being a reliable bench guard. You could say something similar with Pritchard but I do believe his outside shooting will give him the upper hand and be more likely to achieve a long term bench role in the NBA.