To me, saying "it's hard to pin down what the claims for meditation are" is a bit like saying "it's hard to pin down what the claims for psychiatry are". Meditation comes in several flavors and within the flavors is practiced in very different ways. The top level claim is "this will make you feel better" but when you try to verify that you're going to have to restrict it to something narrow that doesn't represent the whole thing.

The self-restraint aspect of mindfulness is one superficial outcome of the practice, one which I think emerges quickly if you engage with mindfulness. It's not the whole thing.