It's not necessarily more batteries that are the problem; it's that to drive electric bikes, boards, and scooters; the battery packs are big. When those things go up, you know about it.

An RC car battery going up is not ideal; but it's a lot smaller. A typical e-vehicle battery would be something like 24 cells on the limp low power budget end to 96+ cells on the scary bespoke end. A typical 'battery fire' is one cell catching fire and setting light to neighbouring cells. Each cell that catches fire is more-or-less unstoppable until it feels like stopping; and lithium (+li-po and li-ion) battery fires give off rapidly expanding clouds of toxic gas. And the cells spit fire, so anything in the same room is in direct danger of being set alight.

It's mostly the toxic gas that'll kill you, though, so grab children and pets and get out the door and away before you have to breathe in again. Most people can hold their breath for a minute from a standing start, so that's how long you've got. Forget about putting out the fire unless you have a bucket of sand standing by (which you should); and even then it's a side mission on the way to GTFO-ing out of there within your minute.