Waterfall is best for projects with concrete timelines and well-defined deliverables. If your major project constraints are well understood and documented, Waterfall is likely the best approach. The Agile methodology was created for projects where the significant constraints are not well understood.

In reality, most projects in the real world are neither totally well-defined or totally without significant constraints. Therefore, the methodology and tools that are able to support the combined strength of both methodological approaches and interchangeable views of the project information is important.

An Agile WBS can be used at the beginning of every short time-box period or sprint. This will help to define each small task, so Agile team members know exactly what they are working on. It can also help with assigning the right resources to these tasks.

You can start by defining a high-level plan with sequential phases using WBS, but incorporate Agile principles such as frequent feedback, iterative development, and adaptability.

Tailoring agile and WBS in subproject level would let organizations take advantage of the best of both worlds to find the solutions best suited to their individual needs.

provides you tools to identify stakeholders and user groups and establish project goals, objectives and timelines to communicate to the stakeholders and users.

provides you tools to organize surveys, focus groups and one-on-one interviews with stakeholders to gather requirements information.

allows you to use techniques like business process modeling and Gantt charts and document precise and accurate requirements to truly reflect the needs of stakeholders.

Human communications sometimes can be ambiguous or vague. It is important to ensure each requirement is testable so that teach requirement is clear, measurable, and complete.

Even for a very small project, it easily has hundreds of tasks, dependencies, deliverables and resources in its project plan.  Each change in one item could generate chain effects to affect many items.  People in the project can't tell easily the magnitude of the impact.  Such a type of project environment is counter to people's efficiency because people know the problems too late.

was designed to provide you with such an environment.

The “too late” environment mentioned in the People Efficiency section greatly affects management efficiency as well. The “too late” effect promotes more lies for defensive reasons and it isn't easy for managers depending on their people but not trusting them at the same time. Moreover, the most common type of lies in a project environment is lie-of-omission.

provides you tools to proactively forecast and plan resource needs for all your projects. helps you to allocate the right resources for the suitable projects at the right time. In addition, helps you track and improve your strategic resource allocation. As a result, resourcing costs reduce significantly while the value increases.

facilitates communication between project stakeholders. Without everyone working in tandem, projects can easily fall into disarray and teams can lose sight of objectives.

straight-thru-processing is having all stakeholders interacting to the same transactional workflow in real-time to minimize the unnecessary human intervention to protect the data for high integrity.

provides you with the evaluation tool that can analyze and explain your project or project portfolio's performance against a particular benchmark. It is used to identify sources of strong or weak performance while a project or a project portfolio is being executed.

actionable messages are interactive messages that allow users to take actions on the project, activity or deliverable without leaving the message screen. 

assists individual team members in managing their activities and deliverables effectively. It allows team members to communicate and collaborate seamlessly by providing a central platform where team members can communicate with each other, share information and collaborate on their work.

helps resource managers to plan and allocate project resources. It provides features for estimating and allocating the right resources with the right skills to the right jobs and utilizing them efficiently for successful project delivery. It also measures every team member's productivity and helps resource managers take proactive measures to maximize billable/strategic utilization.

provides tools for the delivery manager to control the scope, schedule and budget, and  also motivate and mentor the team members, including helping them to apply project management techniques effectively.

allows project sponsors to manage the project launch, see the alignment of the project to its mission from time to time to influence its execution. When the business objectives change, allows project sponsors to manage changes to minimize unwanted impacts.