One-piece post crowns are not recommended because they combine the post and crown into a single unit, making them difficult to adapt and adjust within the root canal and the coronal portion. This can lead to poor fit, higher stress concentration, and increased risk of root fracture. Additionally, they do not allow for intra-radicular retreatment or post modification once cemented. Contemporary restorative principles favor separating the post and core to enhance customization, reinforcement, and long-term prognosis. Hence, one-piece post crowns should not be attempted in clinical practice.

Reference: Principles and Practice of Operative Dentistry, Sturdevant, 5th Edition.