Feather edge or shoulder bevel crown preparations are contraindicated because they provide insufficient tooth structure reduction, leading to inadequate crown thickness and compromised strength. This margin design also results in poor marginal seal and increased microleakage. Furthermore, feather edge margins are difficult to capture accurately in impressions, which affects crown fit. For long-term success, a well-defined margin, such as a shoulder or chamfer, is essential to ensure optimal crown retention, resistance, and esthetics. Therefore, feather edge preparations are not suitable for any crown type.

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