An outboard or I/O has sort of a ratchet arrangement in the lower unit. If power were to suddenly stop, the ratchet would allow the prop to keep spinning, allowing the boat to come to a graceful stop.
On a straight inboard, there is no ratchet built into the drive line. Here a sudden loss of power could be a whole different scenario, the resulting violence depending on how fast you were going and type of boat.
On a fast moving (say 50-60 mph or more) ski/drag boat for instance, you could easily have anyone else in the boat with you sliding forward in an uncontrollable manner, and the boat's nose just looking for an excuse to go under. Clearly, a total loss of power at speed would be a situation to be avoided. You have to be careful just lifting your foot on a full throttle run.