**Core Concept**
The main disadvantage of Posterior Fibrillated (PFM) restoration refers to the drawbacks inherent in its design and functionality, particularly in terms of occlusion and esthetics. PFM restorations typically involve a metal substructure that is layered with porcelain, which can sometimes compromise the overall appearance and chewing efficiency of the tooth.

**Why the Correct Answer is Right**
The primary drawback of PFM restorations is the metal substructure's potential to cause occlusal interferences, particularly in cases where the metal framework is not precisely adapted to the occlusal surface of the tooth. This can lead to unwanted contacts and potentially compromise the patient's occlusal scheme. Furthermore, the metal framework can also cause metal fatigue and lead to porcelain chipping or cracking over time.

**Why Each Wrong Option is Incorrect**
**Option A:** While PFM restorations can be more expensive than other types of restorations, this is not the main disadvantage. The cost is a consideration, but it is not the primary drawback.
**Option B:** PFM restorations are not inherently more prone to decay than other types of restorations. In fact, the metal substructure can provide added protection against caries.
**Option C:** While PFM restorations can be more difficult to repair than other types of restorations, this is not the main disadvantage. The repairability of a restoration is an important consideration, but it is not the primary drawback.

**Clinical Pearl / High-Yield Fact**
A key consideration when designing a PFM restoration is to ensure that the metal substructure is precisely adapted to the occlusal surface of the tooth to minimize the risk of occlusal interferences.

**Correct Answer:** D.