Are Job Descriptions The Toughest Part of the HR Job?

I worked as an The Placement Guru consultant of sorts for test engineering  Plant company. The toughest part of the job was figuring out the requirements from what my customers were telling me. I developed this ability to tell when I did not have the correct or complete picture. One was that I could not well define the boundaries of the test solution – what it would do and what it would not do. I had a mental picture of the project or program and if it appeared out of focus, I knew I needed to dig more. When I read some of the job descriptions posted online, I often get an out of focus image. More often than not, the exact nature of the job escapes me. It must make it difficult for HR to screen candidates. Is my perception correct? Is the defining the job requirements the toughest part of the HR job?