What determines whether a style of meditation can or cannot lead to insight? On YouTube @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 to this question. 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?
Here is the difference: a thought itself is real, but the object of the thought is conceptual. Loving-kindness is real, but the object of loving-kindness is conceptual. 

When thinking, one is really thinking. When thinking of a cat, the cat is not really there, but the thought is.

When loving a person, the love is really there, but the person is only a concept. 

Mind refers to the process of thinking, loving, etc., itself, not the object of the mental activity. When the mind focusses on the movements of the body, the movement is real, the focussing is real, but the body part itself (e.g. hand, foot, stomach, breath etc.) is a concept. 

If one focusses on the breath, for example, going in and out of the body, one is focussing on a concept, as it is not directly related to the observation of the sensation. If one focusses on the sensation at the nose or the abdomen, then one is focussing on ultimate reality.