It really seems like there is no downside to this, other than the minuscule risk of a low-altitude puncture + spark causing a fire, and even there the exposure is small because the amount of hydrogen gas is so much small.

Not to mention that hydrogen is free for anyone who has water and a power source.

and at this scale it seems like the hazards of h2 would be pretty minor. You're not exactly going to have a Hindenburg situation with only a couple dozen liters of H2.

The diffusion is the main advantage of using helium, it takes ~3x longer to leak out, which directly affects flight time.

Hydrogen is actually harder to buy in my experience, helium is sold everywhere for cheap in small canisters for parties, whereas the second requires like, industrial welding suppliers that will want to sell you a large tank for a few thousand or making your own electrolyser and compressor. There's no common use case for it you could piggyback on.