So far, from a technology demonstration perspective, it has become somewhat clear that more degrees of freedom makes autonomous operations easier. road vehicles suffer from terrible decision making in realtime whereas airplanes and drones seem to work sufficiently for cost effective operations. Ships are somewhere inbetween. As long as they are autonomous enough to return to base then maintenance shouldn't be a blocker. Also, the living space requirements on a ship is also a risk because of free surface effects on the interiour.

I think autonomous marine assets are as effective as their engine reliability.