Agree strongly on iron-on. The problem is not that it fails, it is what it leaves behind.
Once adhesive has soaked into the fibres, a proper stitched repair is harder, the fabric no longer moves the same way, and the area stiffens. So the temporary fix has taken away the permanent one.
Inside, as backing under stitching, it is excellent. That combination is stronger than either alone and it is what most durable repairs actually are.