According to Fox,
Words per hour is the first and most important metric in the long run. There are only so many hours in the day, and if you want to make a living with this whole writing thing you need to maximize those hours. That means getting as fast as possible, which is the whole point of this book.
While getting to the end of your manuscript is certainly essential, surely the point of writing isn't to blurt out as many words as possible in a short amount of time?
Fox claims that he writes at least 5000 words EVERY day, totalling about 1.8 million words a year. Yet he also says that he is currently on a schedule for two short and two long books a year (a rough total of 450k words). This means that he allegedly writes four times as many words as he eventually publishes. Imagine writing a whole book from scratch three times, and only publishing the fourth full iteration. I'm not saying this is what his work flow looks like, but for someone who claims to be preaching efficiency, this sounds like a whole lot of lost time and effort.
Overall, I'm just not sold on this way of writing. If you really dig down into it's philosophy, what is left is quite depressing.
The key is constant, incremental progress. Each day needs to be better than the last.
While Fox makes some weak remarks about how it's fine if you have a bad day, but that doesn't take away from the fact that his method will inevitably run you into the ground. There is no such thing as endless growth for human capabilities. You will reach a plateau, one day or another. Stephen King has always been prolific, but he is not getting more and more prolific despite gaining more experience - he has reached the top of his game. Some periods will be slower than others. Some days you might only write 800 words - 800 great words that might be one of the most powerful passages of your book. Yet to Fox, this day would be a failure, because it cannot be put into his spreadsheet.
Despite having tried writing sprints, they did very little for me. All it did was make me obsessively track my amount of words (as I should be, according to Fox), at the expense of actually getting into the flow of writing.