The 'eight volumes' linked to are at the Making of America site (University of Michigan). They are not Child's English and Scottish Popular Ballads (1882 - 1898), but his earlier collection, English and Scottish Ballads (1860), which was a preliminary attempt at the subject with a much narrower scope. See previous discussions (links above) for more on that.

The version presented on the "Sacred Texts" site itself is one of several conversions to html of Cathy Preston's concordance, this time arranged as individual pages for each category, and therefore more easily accessed than most of the other online conversions of her work. It includes, for some reason, the preface to Child's earlier 1860 collection; but has neither the preface to the set presented, nor any of the extensive, and vital, supporting materials (notes, sources, bibliographies, glossary and so on). Since all contextual information is omitted, it is of little use to the serious enquirer, though it may be handy for people wanting raw material who are not particularly bothered where it came from.

It is not in any sense "the entire work", though it's understandable that people who aren't familiar with the real thing might not realise that. It's a pity that "Sacred Texts" were not more frank about the considerable limitations of the text they have copied.

For an annotated list of this and other conversions of Cathy Preston's concordance and related materials, see  http://www.folk-network.com/directory/links/song-collections_books.html.