Firstly, a little context: Wuthering heights is written in two parts, split by the age of the character. While it's still one book there's a fairly clear difference between the two parts, and the first could almost be read on it's own as an independent story. The second half is much more reliant on what happened in the first.
Two groups of people are reading Wuthering Heights. Critics (literally those looking to criticise the work, not in the "movie critic" context, which is really more of a reviewer), and "romantics" (or rather, non-critics, who are viewing the text as a romantic novel)
Critics see the second half of the book as commentary on the first half, rather than an addition to it. At best they see the second half as neutral (just a commentary), and at worst some of them see the second half as undoing the first half. They see the first half as being the "real" work, with the second half being a kind of study of the first half.
"Romantics" (or those who give the second half a more "romantic" reading) see the second half as adding more to the first half of the story, and being a true "second half" of a single whole.
In short, it's saying that the second half of the book can either be seen as worthwhile (if you are enjoying the story and "romantic" about it), or as a bit of a waste that adds nothing, or possibly even makes the first half work.
Note that "romantic" and "critic" as used here are not necessarily the same as the current way these words are used. A romantic reading in this context is more of a "sympathetic to the story" reading, one where the reader wants to see the second half as part of the first, so does see it that way. A critic is not a reviewer like a modern movie reviewer (which we'd call a "movie critic"), but is rather talking about someone who is out to be critical (negative) of the work.