Half-assed happens in both scenarios. If you have something shippable by the pre-ordained date or shortly thereafter, it ships. Whether you use an adaptive or predictive approach, the outcome is the same that further improvements may or may not happen.

The advantage of the predictive approach is better driving of your workers with a deadline. The disadvantage is low morale over time and lower quality products under the hood. Rotating workers who come in fresh can mitigate this outcome but at a cost.