I am not sure how I found this book. I really liked the plot and thought the writing was ok. I can understand why some people dislike the book while others love it. It is not a normal formulaic type of book. In many ways it is much darker than the Hunger Games were. The bad guys are really bad and evil and don't try to hide it. There aren't any "good guys", there are only those who are surviving.
I have since bought all of the sequels. I have shared them with my niece who is an English teacher. She read them and really liked them. Her husband, who is not a big reader, has read them and really liked them.
Yes, they should not be compared to Harry Potter. Harry Potter has a much clearer story regarding good vs. evil. HP has lots of people helping the good side. Tunnels is much darker and the struggle is first with survival and way down the road they may be able to live to be good. There are not many helping the heroes as anyone who gains knowledge of this world are destroyed.