Good question without a satisfying answer. A couple of thoughts, but in no way from an educated person in this area, just stuff I have picked up:
Insurance companies are inherently cautious and conservative. Over time they usually get things right, but often take time to get there.
Not all lithium chemistries are alike in performance and safety. LiFePo4 sounds like the safest, but it is, in many ways, still early days for marine house battery usage.
There have been serious fires and there continue to be. And there continues to be random, somewhat rare, but occasional fires. These occur with the proliferation not only of phones computers (charged on bedding for example), but now high voltage, hi amp lithium batteries for the power tools (drills, vacuums, saws etc.) that are now part of everyone’s tool kit. Poorly stowed on a bouncing around boat, they can short out and get damaged, not to mention the accumulated stress (not designed for) by being around salty air 24/7/265.
Lithium fires are particularly hard to fight, I think.
Lithium has to employ a sophisticated battery management system (BMS) to be charged safely.
Some lithium battery manufacturers (house batteries in a boat’s bank) are flogging their product in ways that have yet to be proven by the test of time.
It is one thing to have a fire in a car or home that you can walk away from: a boat is a different story. There is little that scares me more than a fire onboard.
All vessels should have as part of their regular safety habits a recognition that battery charging of the proliferation of devices deserves a attention. On Alchemy, this includes a review of chargers: probably best to always go with the manufacturer’s chargers (even if more expensive) than the much cheaper charger from a “super-huge” store. Also keep an eye on wiring, especially the joint from the wire to the connector into the device (I often augment the joint with some strain- relief tape on devices that are vulnerable to repeated ins-and-outs or wires that get tugged occasionally). All charging is also done on a hard surface and I have smoke detectors as part of Alchemy’s early fire detection.
All the above said, I think the area of lithium batteries is evolving quickly from a Wild-West free-for-all atmosphere to something much more predictable, safe and every-day. I suspect every year a greater percentage of boats will have lithium batteries.
My best, Dick Stevenson, s/v Alchemy