You've got talent, peak production (i.e., capability) and consistent production. Most teams have a fair amount of talent, and a lot of teams can hit high peak production days. What escapes all but the best teams is the ability to maintain production consistently enough to win at least six out of ten games -- what it usually takes to make the post-season.
While all facets of the game feed into the W/L results, what we're speaking of here with the Cubs is not a peak production issue, nor a talent issue. You can argue that the Cubs have one of, if not the, most talented starting eight in MLB. And they are capable of putting up multiple five-plus-run innings, almost seemingly at whim.
But they will then go through periods, which do not seem to correlate to the overall talent of the pitching they are facing, where the self-same highly talented starting eight scratch for three to four hits and maybe one run. For three to five games in a row, or five games out of seven.
You can have five straight quality starts from your rotation, and lose four out of five with that kind of offensive production pattern. You can then get your starters shelled and pulled after pitching two to four innings, and win five out of six when you get into a hot streak.
In 2016, the club hit a major cold streak just before the all-star break, and both gave up a bunch of runs and stopped scoring that many. They also went into a mini-slump in the World Series, which got them squarely behind the 8-ball trailing the series 3-1 and putting them into a position where they could easily have blown the whole series. In 2017, they had extremely inconsistent offensive production for almost the entire first half, and then hit a major slump in the NLCS. Last year, the slump did blow the whole series.
No matter ho good this team is, if they cannot find a way to limit the dips to one or two game stretches, they will continue to be in danger of hitting a cold streak at just the wrong time that could do anything from blowing out a post-season series, as happened in last year's NLCS, to pushing them out of playoff contention entirely.
Short form: they gotta get more consistent.