The team defensively, has been terrible for the last 3 years. You can develop players, but team defense is a by product of all players on the ice.
There are a number of things I don't like about the entire coaching staff. I think these guys are out to lunch on how they enter the zone, situational play (with leads, without, etc), lines, ice-time, PK, match-ups.
Some of these things are directly on the coaching. PK is on the coaching. Match-ups. Coaching.
So it isn't Ruff alone, but the others too. The thing with Ruff is that he has a track record of bad defensive teams. It has been that way for years. Close to two decades now. That's a large sample size. Has he had good teams in the past? Yes. The post-lockout Sabres stand out. He had one year of MVP play from Jamie Benn that helped him in DAL.
But the Rangers have been some of the worst defensive teams in the last 3 years. It's not just the personnel on the ice imo. It's the schemes, planning and match-ups.
When the year comes to an end, we can point to a number of posts before this season began and look at the outcomes.