Particularly with the narrative songs mentioned earlier, perhaps the litmus test is whether the listener ends up focussing on the song or the singer. It's not so much about what the singer feels about their ability but what the audience feels about the song itself. A friend once reported a comment he'd heard to me which, paraphrased, goes something like "A poor singer stands in front of their song, a good singer stands behind it."   For me the listener should be listening to the song and the singer shouldn't get in the way of that: the words and accents need to work so the listener hears the story not the accent. Different singers may achieve this in different ways with different audiences: I have my own way but I don't think it's a case of one size fits all.