What is the disadvantage of PoW?

As a keen observer of the cryptocurrency landscape, I'm curious to understand the limitations of Proof of Work (PoW) consensus mechanisms. Could you elaborate on the primary disadvantage of PoW? Specifically, I'm interested in how it relates to energy consumption, scalability, and potential for centralization. Many argue that PoW is overly resource-intensive, leading to significant environmental impacts. Additionally, with the ever-increasing number of transactions, does PoW struggle to maintain scalability? Finally, are there any inherent risks of PoW fostering a more centralized ecosystem, rather than maintaining a truly decentralized network? I'd appreciate a concise yet thorough explanation of these concerns.