From a fleet perspective, Boeing has not identified and resolved the root cause of the 787 electrical fires. Whilst the battery 'box' has been approved and the aircraft flies, the necessary elements of combustion remain.
Often repeated, but simply not true. Boeing completely redesigned the battery and the charging system. However, due to the extent of the damage to the battery during the two battery fires, they were unable to firmly establish what the root cause was. So using a belt and suspenders approach, they put the battery in the steel box.
Before the redesign, 40 787s experienced 2 battery fires in about a year.
There are now ~800 787s in service, averaging ~4000 hours/year utilization (that works out to over 3 million flight hours/year fleetwide). Since the redesign there has been one battery event - a single cell failed but thanks to the battery redesign the fault was contained to a single cell and didn't propagate to the other cells. The cell fault was traced to a manufacturing defect and corrective action was put in place.