What Is the Trust Trade-off When Using a Private Mempool Service?

The trust trade-off is that a user must trust the private mempool service (e.g. the relay or block builder) not to front-run their transaction itself. By submitting a private transaction, the user reveals the profitable trade details directly to the trusted third party.

If that party is malicious, they have a clear opportunity to execute the front-running attack themselves, bypassing the public mempool entirely. The user is trading the risk of public front-running for the risk of private, trusted-party front-running.