1) Agile as a silver bullet - Yes, agile methods can save time, increase business buy-in, and create a high quality product, but they are no silver bullet. They will not bend space or time and allow you to deliver more work that is possible within the constraints of limited time, budget, and resources.
Deciding to switch a doomed project to an agile approach will not make it succeed. You may fail faster, or at least discover realistic progress indicators (velocity) earlier than with a traditional approach, but unachievable goals will remain unachievable. By all means use an agile approach to salvage a core set of valuable functionality from a failing project, but don’t expect agile methods to miraculously make the impossible possible.
2) Agile as an excuse for no discipline – Contrary to some people’s beliefs, agile methods do not abandon discipline and jump straight to coding without the need for plans and estimates. Agile methods involve many high discipline activities and techniques like Test Driven Development, Wideband Delphi estimation, and bi-weekly iteration planning and estimation sessions take a lot of discipline.
If team members are pushing back on estimating or cannot explain the release plan, these are warning signs that they may not be following the agile practices. Instead they could be using agile’s preference for low ceremony documents as an excuse for avoiding doing the disciplined activities that make up each of the agile methods. Hold them accountable, ask for their estimates and request their retrospective findings.