On the large fan/high bypass engines, reverser operation is more fragile than with older engines. They are more prone to jamming, especially the widebody engine types. Once you have even idle reverse, you cannot assume it can be just cancelled and away you go. Translation from reverse to forward can also take varying time. You just don't have the time available to cancel reverse power, wait for low thrust, translate to forward idle then apply power. Once you select even idle reverse, you are committed to stop, whatever happens.