In general good quality 18650's or similar form factor cells are fairly safe, it's the lipos that sometimes worry me as they keep trying to make devices thinner and thinner and that is pushing the tech beyond what is reasonable. I don't understand this obsession with making stuff so thin. Anything under like 1cm is diminishing returns really. I'd rather have a thicker device with more IO ports and a bigger battery. Especially a laptop. A phone I can sorta understand, but even then. That said, safety wise it's been pretty good other than the oddball incidents like the Note 7. That was not really related to thinness just some kind of fault where it would short out.
I think they should just start putting class D extinguishers on planes and even in schools/office buildings for when someone's device decides to Allah Akbar itself.
On the other hand vapes are probably more dangerous in general and use 18650's, but lot of people get the batteries off Amazon/Ebay and they are fakes are sometimes more dangerous, then they are putting in coils that are a fraction of an ohm, which is basically a short circuit. Yeah, it's going to explode.