A book review is nothing more than emotional snapshot of how an author’s work speaks to the reviewer.
Anyone can go through a book and mark syntax or grammatical errors. That is the purview of elementary educators. What we expect reviewers to do is to read a work and tell us whether the devices the author used worked for the reviewer. Do the analogies seem out of place? Is there too much ambiguity in story. Can the protagonist be understood adequately with the given backstory?
And these understandings of the author’s work is subjective on the part of the reviewer, and the subsequent review is then only useful to someone with similar taste and understanding as the reviewer has.
Many things are more subjective than we consider, and we are often more opinion-based than we acknowledge. Remember that when making decisions.