The thing is heat from the outside gets moved inside of the house using a heat pump, and to facilitate this movement you need somewhere between 1/2 and 1/4 of the energy you end up moving. E.g. a heat pump with (coefficient of performance) of 4 would move 4kW of heat into your house and use 1kW of electric energy to accomplish this. Gas by comparison moves 4kW of gas to your house and burns it there to get 4kW of heat.
So you could burn a bit more than 1kW of gas in a modern gas electric plant, turn it into electricity and use it to run a heat pump and you would end up emitting less CO2, the real world grid might skew that worse because generally you don't end up burning coal to heat housing but you might still use it for electricity. So generally even though it might be unintuitive the more complicated and lossy way to heat your home (the heat pump powered by fossil powered electricity) , is the more effective one compared to burning the same fossil fuel directly because you use the heat pump to capture heat from the environment.
Probably because trains are limited in both weight and volume compared to ships and also less efficient. If you have this short route and know it'll need this amount of cargo shipped it likely makes sense.
This single ship can carry more containers than any train could be expected to pull, likely by at least one order of magnitude.
All in all I'd guess the advantages are roughly:
Disadvantages:
I would also not expect the risk for catastrophic fires to be all that high. This ship has the batteries be containers. So once you've designed a container that is a large battery, you've already spent so much that a proper BMS including proper battery fire suppression as well as proper breakers/contractors are things you've built into it without even thinking about cost. The separation provided by building containers as the battery is the next line of defence if one container fails spectacularly, it also allows the batteries to be maintained on land, much cheaper than if they were part of the ship.