Certainty is low because whether the clause is enforceable depends on the amount of actual damages. That amount is often difficult to predict since it depends on the exact circumstances in which the breach occurs. To the extent those circumstances are difficult to predict, actual damages are difficult to predict. Indeed, parties write liquidated damage clauses in part precisely because expectation damages are difficult to predict.
Courts that follow the minority rule avoid excessive over- and under-compensation at the cost of making liquidated damage clauses far less useful as a device to reduce uncertainty about breach of contract liability.