LFP is a lot safer than NMC. I think it's almost on par with Lead-Acid.

Weirdly, none of the many phones, tablets and laptops I've owned have ever caught on fire.

I guess I've just been lucky.

The acid in lead acid is sulfuric acid and if overcharged vents hydrogen gas, thats why they need a ventilated space typically. Sealed lead acid have safety vents that might pop if enough pressure builds.

They are most certainly not inert, they just have well established safety and charging protocols and are not used in very high quantities together because of their low energy density and cycle life.

LFP batteries which have iron phosphate cathodes are very stable compared to colbalt based batteries that tend to have catastrophic failures due to overcharge causing cathode failure. LFP have higher cycle life and are cheaper and typically whats used for storage and application where the loss in erergy density is not a big deal.