Take the delta of his xGF and his teams xGF, take the delta of his xGA and his teams xGA. If you are wondering how to find them, feel free to look at EH, that is a very nice site for things like this.
Take the difference between his xGF with the team and the teams without him, and he is, again, bottom 50 in the NHL. This means that the team is expected to score more goals without him than with him. With a top heavy team like new york, you'd expect that. Panarin, Zibanejad, Kreider etc will be the biggest threats, and will likely throw in the outliers, so I can see why you may not like it offensively.
However, you said yourself, Buch is bottom 60 in xGA. When you compare that to what the Rangers are without Buchnevich, they numbers are so contrasting that saying he is one of the worst analytical defensive players in the league is not out of question. You can find the team numbers on
www.chartinghockey.com, or find a comparison on corsica.hockey. Both extraordinary sites for analytics. However, neither has the evidence to contradict what Buch's liabilitys are defensively. Also looking at raw stats.... he was on the ice for more goals against than all but 5 of his teammates? (Per Fox Sports, very good for the depth base stats. All of this combined contradicts the RAPM stats. You can say it's based on his ice time, but it's not like it's a limited sample size, or have you forgotten what that means?
So tell me again, where was my credibility in question?