Brian Cashman has been the New York Yankees' general manager since 1998. That is a long time to be in one place in any line of work, let alone one that is so public and demanding as the GM of one of the most famous professional sports teams in the world (especially when his boss was, well, The Boss).
Since no one is perfect, it is natural to expect mistakes to be made along the way. No matter how hard decision makers try, it's inevitable. Cashman is no exception. Like every GM, there have been trades and free agent deals that have been successful and those that have failed.