What Is the Risk of a Zero-Fee Transaction Being Rejected by Most Miners?

The primary risk is that the transaction will remain unconfirmed indefinitely and eventually be dropped from the network's memory pool (mempool). Miners are economically rational and prioritize transactions by fee-per-byte to maximize their immediate profit.

Since block space is limited, a zero-fee transaction offers no financial incentive and will be passed over for any fee-paying transaction. Furthermore, many network nodes are configured to not relay transactions below a certain fee rate, meaning the transaction may never even reach a miner.

This effectively makes the transaction unbroadcastable and unconfirmable in a congested environment.