For the navigation part the technology is probably well enough advanced to where remote control and even autonomous operation can be implemented now, but the legal aspects are lagging behind.
Propulsion machinery able to reliably operate for weeks without service is also in existence, but all the auxiliary equipment required to keep it going is maybe less reliable.
To go for a hybrid solution, where there are some engineers left on board to ensure that those systems works, is not a viable alternative, since that would also require all the same “hotel” services as on today’s ships.
The alternatives, such as Gas turbines run on LNG/LPG to supply propulsion power, directly or via electric motors, are not economical when compared to low speed diesel run on HFO.
But Hydrogen fuel cells with enough capacity to drive a mega container vessel (60-80 mW) is under development and will be available by the time autonomous ships crossing oceans becomes legal.
What happens if an autonomous ship have a breakdown in mid-ocean?
The same as if a manned ship have a breakdown. Drifting until a tug gets to it and manage to hook up tow. (Emergency towing arrangement compulsory, regardless of type/size)
What if it breaks down close to a coast? Also same as a manned ship. Anchor(s) can be dropped remotely, if necessary.
What If an autonomous ship have a collision, or goes on the rocks? Nobody need to risk their life to save the crew, since there is none. If Hydrogen powered there is no pollution. (Unless from the cargo)
What if there is a fire? Automatic or remote activation of Water Mist or CO2 release. Machinery shuts down until salvage tug with fifi crew arrive. Nobody in harms way.