I would imagine the statistics are probably low; especially compared to ice engine fires.

However....

If Ebike batteries are going to combust there is a good chance its going to be while charging often inside a home.

If your 20 year old clunker has a fire its most likely going to be on the side of a highway.

So even if ebike battery fires are rare the chances for catastrophe when they do happen are high.

So although I'm against fear mongering I do think they warrant extreme caution.