The only outcome stat for an offense is runs scored. There are many stats the help describe inputs to how many runs a team will score, but in the end, all the only outcome that matters is how many runs you score per game.
The cubs currently rank 14th in the NL in runs per game.
The question is what's contributing to this anemic result. None of the traditional stats, nor new stats really shine a light on the problem:
Stat: NL Rank
None of these stats, save one, is as bad as the Cubs RPG. Can grounding into double plays and/or lack of stolen bases really have this dramatic an effect on runs scored? Granted, the Cubs don't do anything particularly well offensively this year, but they don't do anything as poorly as scoring runs.