Now, OTOH, your concern on LI batteries has some merit. Personally, not too keen on them, blew the nose off a MIL jet I was flying once with a BTR, lots of metalwork and a new engine needed, so cannot say that I am a fan of these.
First of all my rant is not against the airframe itself. But the choice of battery of this size and technology makes me a bit nervous.
Even though my comment was maybe a bit over the top, I have personally seen 1lbs proper brand LiPo cells go off which were charged with a proper brand charger. It is quite the fireworks. And there wasn't any reason to expect it. Went suddenly boom during charging. Took 4 litres of water to put it out. Water works fine against urban myths btw. I wouldn't try it in a closed room though, it creates a bit of poisonous acid.
From all what I've seen Li Ion is pretty much identical in behaviour.
70 times the size of this fire I do not want to imagine. And especially not inaccessibly in a carbon fiber tube at 35kft. These cells have self ignition capability once charged beyond a certain level. Be it due to a failure in the charger or an uncaught manufaturing defect in a cell or a pre- existing damage due to handling or temperature exposure. (There is just a very thin plastic foil separating the layers - you damage the foil at any place - you light up your place big time).
You get the point. Too many variables which are hard to control and in case of failure there's not much of a mitigation plan.
Just good ol' luck might save your day.
And all that while there is an alternative with only limited drawbacks.
Just one word: Why?