I've read many books in my life that either started off awesome, developed well after a rough start, and then failed around the end. It happens a lot with Stephen King. But I don't know that it's a reason not to read a book. People want to know ahead of time, by reading amazon reviews, that a book is going to be fulfilling. There is no more mystery in reading a book. If 20 people on amazon reviews say the ending sucks, then the person will not take the journey of reading the book and determining for themselves. I devoured Freedom when I first bought it. I was enthralled. But he didn't pull it together. If you'd asked me half way through the book, I'd have told you it was great. After finishing it, I'd have told you it was a waste of time. Still, this is a journey you should make yourself. People who want to have a decision made for them read these reviews and don't even make that initial journey.
I feel for the authors.