I agree that it's fun and can often be safe if built correctly for the right application. But I think UAVs don't experience the same beating and vibrations that ebike batteries do, which I think is what the bigger problem is with ebike batteries in particular. Unless carefully built, things like balance wires can shafe and cause a short, the spot-welded tabs can break off or the connection resistance increase, leading to hot spot. The BMS can have hot spots (FET based ones in particular) that lead to failure or electrical fires over time. Humidity mixed with various electrical potentials on nearby PCB traces causes corrosion and eventually failure of the BMS, etc. And it's not all the BMSes that are built the same, they may all have the same set of features on the happy path, but they're not all created equal when their PCB starts failing and fail-safes are required.

Basically in a "tough" environment like ebikes, I think the aging of the pack is much accelerated, which leads to lower mean-time between incidents. Combine that with the profile of the average buyer (wants the cheapest, longest range battery they can get) and it's an even higher probability.

Don't get me wrong, I absolutely love custom DIY packs. It's just way more dangerous than meets the eye, especially in dirty and high vibration environments.