Is the author writing badly just for fun, then? Is this potboiler going to turn into something thought-provoking after the first chapter of cardboard characters and cliché exposition? That wouldn't be a bad idea, like an updating of "If On A Winter's Night A Traveler" with riffs on modern fiction styles, but I doubt "The <Greek Letter> <Ominous Noun>" ("The Omega Directive" "The Alpha Sanction") is going to turn into into a parody of airline novels after starting out as merely a poor example of same.
There are plenty of novels that change quite significantly in character partway through. There are plenty of films with a weird first act before you get into it.
If a book is popular and has thousands of people reviewing it, why not let the people with enough context (i.e. those who actually read all of it or at least the majority of it) review it? Is one so smart that their review based on only partial information is necessary and needs to add to the chorus of all of the other reviews?
Would you trust the review of a professional movie critic if they said well they didn't like the first 15 minutes so they just quit. Well today because we all have a megaphone through review sites, we are all on the level of professional critics to more of an extent than anyone would be in 1990 so we all have some responsibility to write informed reviews.
Just to provide an alternative point of view from a reader, this is fine behavior. So long as you mention this in your review.
I’ve DNFed both movies and books in 10% or less. And that’s ok. There’s a ton of excellent books and movies to justify pushing yourself through content you don’t like. As someone put it, we’re not looking for needles in haystacks, we’re looking for needles in mountains of needles.
So, it’s not personal, but a review at 10% is still a valid review, if it’s honest about the slice read.