What determines whether a style of meditation can or cannot lead to insight?

On [YouTube](https://www.youtube.com/watch?v=GDDY4gOexVA&feature=youtu.be&t=2m14s) @Yuttadhammo seems to say that tranquility meditation requires focus on a concept and as a result -- i.e. because a concept is not "real" -- that meditation cannot lead to insight. And, by way of contrast, he says that the rise and fall of the abdomen *can* lead to insight because the abdomen (and its rise and fall) *is* real. Is that -- the nature/reality of the meditation object -- the primary difference between meditation intended to achieve tranquility and that intended to achieve insight? But if so, how does that tally with Y's [answer](https://buddhism.stackexchange.com/a/8117/199) to [this question](https://buddhism.stackexchange.com/q/8080/199) . There, he notes that the Visuddhimagga allows for insight to be achieved by scrutinising “mentality” (nāma). But how is mentality real while a concept is not?