Because amountOut = amountFeiIn; and amountFeiOut = amountIn; there is no slippage, and no risk for any front-run.
The check below is always true for that reason:
SimpleFeiDaiPSM.sol#L54-L55
A better require would be to check if the balance of the token that will be transferred out is sufficient to avoid a revert on the low level balance subtraction.
