It seems like it's just a fundamental rocket equation type limitation, battery tech has a very low mass vs energy density, and it's paired with an aircraft that

It seems like it's just a fundamental rocket equation type limitation, battery tech has a very low mass vs energy density, and it's paired with an aircraft that will always be much lower efficiency than fixed wing. I'm sure much longer flight times could be achieved using combustion engines, but obviously that wouldn't be a great idea inside a mine or in sensitive areas. One workaround could be a sort of blimp/dirigible, but I don't think it could be scaled down the same size as a quadcopter.