I normally don't post questions like these, but this came up in discussion today.
To be clear, I hate hoverboards, and the people who use them (at least on campus) tend to either be oblivious nuisances to straight up dangerous. A girl was riding one indoors today, in class hallways, and nearly ran into three people while on her phone, and wasn't even aware of the three people diving out of her way (the reason discussion came up)
That said... Hoverboards have been recently deemed unsafe to use. Several major companies have outright said "Do not use our product anymore". A number of companies have faked UL certification. Amazon no longer sells them at all. In light of them blowing up and burning down property every other day, I still see people using them.
Would you stop someone riding one to let them know it's dangerous? Or let them go on their merry way knowing that it might be entirely possible (but probably small chance) it might ignite at some point?
I personally have not done so, mostly because half the time they probably wouldn't even notice someone flagging them down. Let alone believe that their company has made public statements.