There's a fair amount of peer-reviewed research on the benefits of mindfulness meditation, and it doesn't seem all that unlikely that exercising your brain in different ways really can help your brain function differently.
How might someone conduct a blinded placebo controlled trial of a suicide prevention campaign, or of parental advice to help reduce risk of cot death, for example?
They wouldn't. They'd roll up their sleeves and get deeply involved in the messy, unpredictable world of human emotion and social interaction.
And that's good and appropriate, and there are effective and responsible methods for doing it.
Just because we can point to something and say "aha, that's subjective" doesn't mean we need to eliminate it. In fact, the more we try to eliminate it, the harder it becomes to translate evidence from the lab to the real world.
Objectivity world be nice, but it's vanishingly rare.