Problem based learning (PBL), though recognized as beneficial to student development, faces implementation challenges in engineering education. We conducted a Delphi study with PBL and (aerospace) engineering domain experts to understand challenges around learning outcomes, problem design, facilitation, and assessment. The context for the study was an introductory aerospace engineering course, transitioning from a traditional lecture to a PBL format. We found consensus among both expert groups as it relates to ideas about learning outcomes. However, with respect to problem design and facilitation, we observed a slower and more contentious convergence, with some ideas failing to reach consensus. From this, four salient issues emerged as potential barriers to PBL implementation that supports students in important aspects of their professional development: problem framing, making relevant connections to society, a deficit view of students, and discomfort with facilitation. Implications related to these issues are discussed, with consideration given to complications stemming from the isolated, individual-course implementations of PBL commonly found in practice.