Well, as you ask, writing in 1958 he mostly made quite a good job of it, as he had a limited number of texts at his disposal, so his comparison of these texts (p45-54), if a little laboured, is reasonable and can't be argued with. However, his following of the romantic line on the meaning of 'foggy dew' (p54 to 57) is somewhat naïve in my opinion. To be fair to him he does state, 'as it stands the song is nonsense' and refers constantly to the 'confusion' and at one point even suggests quite correctly that the title may be a corruption of something else. BUT why follow the romantic line, picked up on by others
and regurgitated? We now of course have a much fuller picture of what the song meant, and it can be argued that he did his best with the material he had. As you say he presents a detailed study of a few of the songs but these are of little use today as we have much more information and many more versions to play with, and we know much more about the evolution of individual songs.