The stuff seems relatively safe, but there are some considerations since it is a eye and lung irritant (MSDS here). As a side note, it is nice that they have an MSDS for the stuff, considering most thermal compounds from China that I found on Amazon or Ebay don't even mention one (we could probably learn something about deregulation from China ;-). To be on the safe side, I took the printer outside for the first heatup cycle after installation, and ran a PID tune outside as well, since it was mentioned that "Initial exposure to high temperatures evolves, in small amounts, fumes that may be irritating or corrosive to the upper respiratory system." Though I suspect that "high temperatures" for this stuff is really a lot more than anything my printer is capable of - like 600°C probably. At 240°C which I started it up at, I'd guess it may have boiled off some water since it is 65-70% water. The dust is however an irritant so when swapping the heatbreak or removing it from the heatsink, I would probably wear a face mask or respirator to avoid any dust.
Update 9/23/2021: I was gonna use the Boron Nitride again to rebuild a hot end, and found it was all dried out. It's not super surprising but still disappointing that it did not last in storage. For the record, it was capped tightly in a bag. I don't think for a single use it is worth buying this stuff, and will stick to using the small one time use packets for my printers.