This is very true. it is VERY difficult, because most of the work of Product Management is NOT directly related to outputs like sales or marketing pipeline. Product Management is business management at the product level - or at least it should be. It should not be delivery management, which is what it is in a lot of companies. The work of Product Management is mostly early stage work - before anything is built, marketed or sold and so the metrics are difficult to tie into most output metrics a company has. This is a problem that has yet to be solved adequately.