The author is correct, in the sense that over the lifetime of a software project 90% of the value from unit testing will come from 10% of the tests. The problem

The author is correct, in the sense that over the lifetime of a software project 90% of the value from unit testing will come from 10% of the tests. The problem is that you won't know in advance which 10% that's going to be, so you have to write and maintain all of them.