The underlying problem with lithium ion cells, imho, is that they pack an enormous amount of energy into a tightly confined space. If anything should go awry, that sudden release of that energy, in such a small volume, can have dire consequences.
In order to render them safe, high manufacturing quality is a must. Even then, there are strict constraints on charging circuitry, mustn’t charge too fast, mustn’t over-discharge, etc, else problems can arise. It is probably safe to assume that the likes of Apple and Samsung etc, are highly dillegent in choice of suppliers and manufacturing plant, as well as circuit design. I therefor do not worry unduly about leaving my iPhones on charge. Yes there is a risk, but the risk seems acceptable.
But I would not want to leave an unknown brand on charge unattended, and I’d be choosy about storing it, away from combustible materials. For appuratus that must be left unattended, I’d simply prefer a different technology; my DECT phones have NiMH batteries, for example. I don’t totally avoid Lithium, I recently got a lot of fun out of a sub £20 RC toy helicopter - but I never left the room while it was charging.