Rielly doesn’t really add anything to the power play. No creativity, no shot, nothing really. Sandin or Liljegren would be better options than him.
While he may not be the best at running the PP, over the last 5 years Morgan Rielly has averaged the 22nd most minutes on a PP in the NHL for a defenseman, while scoring the 15th highest number of points for a dman during that time. While averaging 2:45 TOI/PP/G he's nearly 30 seconds shy of the average number of minutes/PP/G that the 14 players who have scored more points than he has get/G. On average, Rielly scores .24 points/g on the PP. By comparison, Chris Letang (12th in scoring by a defenseman over the last 5 years) averages .266 points/g on the PP while averaging 37 seconds more PP time/g. Roman Josi, 4th overall: .3 points/g on the PP while averaging 28 seconds more per night.
The real standouts are: Makar (.43 points/g, 1:07 more time), Q. Hughes (.38 and 51 seconds more time), and old man Hedman (.4/game with 36 seconds more...also number 1 during that 5 year period).
Pietrangelo, Suter, Theodore, Chabot, Morrisey, Werenski, Ekblad, Hamilton and Jones: all have fewer points during this time frame, all with a comparable number of games played, all within 20 seconds of TOI (+ or -) save Morrisey who is averaging 29 seconds less/game, having played 9 fewer games than Mo, and sits 17 points behind him @ .19 points/game.
This suggests that the margins are pretty thin unless you've got a real talent on your team. Part of the Morgan Rielly can't contribute on a PP syndrome, is, I believe, largely based on familiarity with the player and hoping for more. As it stands by points and minutes, he's doing pretty darned well in comparison to many in the league.