I’m assuming this is some biological phenomenon and not straight-up magic, in which case we’d eventually find a way to identify and suppress it. Or harness the chemistry in some way.

Also, given that it’s biological, there will be some people - how ever small a fraction - who don’t combust upon brain death. Those people will cause all sorts of problems because we won’t be sure if they’re dead or not, at least not until they start to decompose.