Setting homes on fire seems like the highest risk -- there's no way standard domestic insurance is paying out if they know the fire is caused by business activities.
This seems like the biggest risk to me. Charging untold amounts of batteries that have been used/abused in unknown ways in my house while my family sleeps doesn't seem like a great idea.
Seems like a great idea to offload risk if you're Bird though! All those hoverboard battery explosions a year or two ago (and for example the FAA baggage bans that came as a result) really soured the image of chinese battery electronics.
Defining the heuristic now: the scooter market will truly be "mature" once Bird et al start incorporating commercial fire insurance add-ons into their charger programs, kinda like how Uber started providing commercial driver and liability insurance.