What Is the Main Drawback of Using a Private Transaction Relay like Flashbots?

The main drawback of using a private transaction relay like Flashbots is the increased centralization risk and the potential for censorship. By submitting transactions to a single, trusted entity (the relay/block builder) instead of the public, decentralized mempool, users are essentially relying on that entity's integrity.

If the relay decides to censor certain transactions or favor specific users, the system's censorship resistance is compromised. This trade-off is accepted by many users as the cost of avoiding immediate MEV-related front-running.