No, it is not overrated. The author is just using a not-so-handy definition of 'unit'. When a 'unit' is a single method/class/function 'unit' testing is not in general a very wise thing to do. In most cases it is better to unit test a set of related classes/methods/functions as a whole because the communication between these things also needs to be tested. In that case it is quite impossible to overrate the importance of unit testing. I also don't think the definition the author uses is the original definition of 'unit'......