Victor Katz's review of the Companion Encyclopedia was in Historia Mathematica vol 24 (1997), pp. 445-452. Responses from Ivor Grattan-Guinness and from Jeremy Gray appeared in vol 25 (1998), pp. 94-97. Katz's critique seemed in essence to be saying that it should have been a different sort of book, while Grattan-Guinness and Gray pointed out that it succeeded in what it set out to achieve, namely to convey the range and variety of past mathematical developments and to provide bibliographical leads for those wanting to take any particular matter further.
I tend to side with Grattan-Guinness and Gray on this matter. The world of historical scholarship is richer and better for the existence of the Companion Encyclopedia. Of course if Routledge had been willing to finance a book of 3600 pages rather than merely 1800 pages then each article could have been twice the length, and the book would doubtless have been even richer and better. Would that we lived in such a world!