The fine line I’ve learned to tread as a reviewer is separating the author from the book. That means when you write a review, you discuss the book and whether or not it holds together on its own merits. The minute you start saying things like:
a. I expect more from this author,
b. This author can do better,
c. This author is wasting their talent,
etc., you run the risk of making the review be about the author instead of the book.
The more reviews I write, the less personal they become. Though comments on what the author could have done seem like an attack, they’re also a sign that the reviewer cares about the author’s future, perhaps too much.
I agree with you that we should only review the book the author wrote. We may speculate the author can write any number of things, but if he never publishes these “dream projects,” it’s simply wishful thinking, like saying, “Lee Child could write a James Bond book.”