I think it's in the introduction of Vol. 2 of that Bronson has a sentence to the effect that he did not feel obliged to include the versions of J.J. Niles. He does not discuss the reasons for this decision, but one may infer that he shared the widespread suspicion that a number of them were extensively retouched or even just composed outright by Niles himself. This is a major issue in a work such as Bronson's, of course. In other contexts it makes less difference. I suppose we will never know exactly how much the ballads may have been altered or invented by Niles; in any case they include a number of marvellous melodies.