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