The biggest strength that traditional requirements gathering techniques have is their structure. When using these methods, the Requirements Engineer (RE) is applying a formula, pure and simple. There are outlines to follow and forms to fill out, which makes the job of RE more accessible to a larger group of people.
Agile requirements gathering techniques throw that all out the window. In fact, they probably get rid of the window, too. This is not inherently bad, but it causes a drastic shift in the pool of people that are capable of pulling it off successfully. In order to be successful at agile requirements techniques, you have to have unparalleled critical thinking and listening skills. You also have to be very motivated and disciplined. Basically, you have to be a rock star worker.
Traditional methods have also been around the longest, and there is a substantial body of case studies showing their reliability. There is more risk in choosing agile requirements techniques because we don’t yet know all of the pitfalls and best practices. For example, one problem we have discovered with agile requirements gathering techniques is that they have difficulty capturing non-functional requirements.
The reward pitted against all of the risks of agile requirements gathering is a dynamic requirements base that can change freely and deliver faster turnaround on changes(due to less overhead). This promises stronger requirements on a shorter schedule, and that’s exactly why people take the risk. All the same, it’s not the kind of risk that should be taken lightly. Make sure your team is skilled and disciplined enough; make sure you’ve got rock stars.