As mentioned in the Nature article and in another poster's comment[1], MRNA vaccines have been known for a while now (decade+ ?), and studied on animals, with some limited phase I / phase II human studies. But nobody's gone through a full phase III before covid hit, so that kind of a wide fan-out is new and exceptional because of the emergency nature of the disease.
But I don't think they're unconcerned. I think it's a combination of lack of decades-long longitudinal studies, and also that previous phase I / II studies haven't throw up significant red flags.