A write up on spicy pillows, thermal runaway and some general fire safety advice
Coming in on this from a fire science and fire safety background. I originally wrote this for someone concerned about their spicy battery in an emulation handheld, I've tried to make it more generic as most of the information is, but there may be some specific references still. Happy to provide sources and the like if needed.
Batteries swelling is a result of thermal runaway and can happen with any lithium ion or Lithium polymer battery, it swells as a safety feature basically before exploding. The swelling is just a build up of gas from a chemical reaction inside the battery due to the breakdown of the protective layer inside. As it continues more of that gas is produced and so it keeps expanding, the force coming from that expansion is generally going to be enough to cause the shell of the device to also bulge. It'll vary between devices, but as the shell on most won't be that resistant to a constant force applied it'll just generally also bulge out. Pretty good that it does, if it doesn't then the gas isn't going to stop coming it'll just be under more pressure so likely to have a larger eventual reaction.
The initial reaction to produce the issue can happen for numerous reasons, it can be cheap batteries but generally most are manufactured pretty identically nowadays especially for these kinds of devices. Cheap materials and the like, will increase the chance of a manufacturing defect. Thing is most companies will be using the same cells to make their batteries, very few will order bespoke made cells unless they really have to. It'll be down to how they're treated. Any kind of physical damage can potentially cause it, overcharging, overheating, pressure, etc. Those all will also affect each other, and be affected by aspects like the age of the battery, number of cycles, etc. The technology is improving but it is still extremely dangerous as a technology, however it's also the most convenient for us hence why we use it.
Heat is the biggest issue for them, as outside of a nail or something penetrating a cell. Heat is what results in thermal runaway and the eventual spicy pillow. The heat of the device in use won't generally be bad enough to trigger the swelling itself, especially before various safety measures would kick in like the device throttling. However heat is going to lead to the cell degrading faster, increasing the risk of thermal runaway occurring.
Manufacturers being at fault for the batteries is a difficult one, this is mostly in regard to phones, handheld gaming devices and the like. Where the device experiences a large amount of power draw, heat, power cycles and the like. Many do pack a lot of power in the device but all they did was make a device they didn't say what could be done with it, how to use it, played on it, make the emulators, optimise it, etc. Sure they hint at it but that's all, if it was a 3DS or Switch then it'd be a little more cut and dry (they do also get spicy pillows but slightly different story).
As for not charging the device overnight that's general advice you should follow for all lithium ion charging, do many people, no, do I, not really. It's convenient not to follow it that's why. However as said before all processes that a battery does will create heat, as it discharges it creates heat, however it also is generally going to be surrounded by external heat sources like the other components. Then as it charges it creates heat, and generally quite a lot more internally, especially when higher amounts of energy are involved like fast charging, but also if incompatible technologies are used. Thing is if it's discharging it means you're using it, if it starts to heat up dramatically you're going to notice it and hopefully stop using it. Charging it, that's when you're not paying attention to it, maybe you're sleeping, or it's in another room if that starts heating up rapidly no one will see, the heat will continue too build internally until, boom. It's why even though I charge my phone while I sleep it's on the other side of the room from my bed, nowhere near any of my ways out.
As for why you see it with certain devices and not others, I can't give a definitive answer. I think there's way too many factors at play, and not enough information available. There's the whole idea people will post about negative experiences more, so likely to see any issues. Availability of information vs older devices like the DS where there wasn't an easy way to share information as easily, and see what others share. Then the combo of once you know about an issue like this you'll be much more likely to spot more posts about it, and likely to remember them like the Baader Meinhof phenomenon. Then just the devices themselves, some will cool better or have more inbuilt protection especially with bigger brands as they have way more to lose if they don't, especially with total devices sold. These are still niche, especially vs say the GBA SP. The Thor sold around 100,000 from a quick Google, vs the GBA SP with 43.5 million so the Thor is around 0.3% of the SP. So Nintendo knows they need to do all they can to keep that thing safe because so many eyes will be on them, if something goes wrong it'll be everywhere. Ayn, it gets posted on here and a few other sites / discords and that's it. Even if there was a death or something the reaction would just be hugely different. So many people have died over the last few years due to battery fires, they don't get that much coverage nowadays especially in the countries they happen most often like China where these devices are more popular. If it happened in the UK say it'd get looked into if it was an Ayn or Anbernic but it wouldn't get the coverage vs Nintendo, or back when Samsung had phone battery issues.
Finally is it's also just a lot of luck. Cells and batteries are some ions moving between two surfaces, through around four substances and that let's us play Diddy Kong Racing in an Airplane. Your devices lasted and someone else's won't have, doesn't mean yours couldn't go tomorrow with no changes at a level we can see. Once the internal reaction can take place, it will, then the issues start. To reduce the odds of that happening, the first thing is to follow any charging instructions, then don't use it since the minute you do those odds increase and as said before unfortunately most things you want to do will also increase the odds of it occurring. Don't let it get to any extremes of temperature so hot or cold, the main thing is to keep it as cool as possible in general within those safe operating temperatures. So yeah more demanding emulation is going to increase the risk of it happening, impossible to say how much you just have to accept it really like many things in life or you won't do anything. When charging have it on a hard surface out of sunlight and away from other heat sources. As mentioned try and charge while you're awake in the same room, if you want to do it overnight, avoid charging it where it'll be a bigger hazard if it does catch fire like a hallway, by a door or exit or near a living being, like the nightstand next to your pillow.
To briefly say the smoke and fumes that come of these is incredibly toxic excluding the fact it's also easily ignitable. You don't want that anywhere near you, those you a are about or your home. Depending on a few factors but generally avoid just leaving anything on charge constantly, especially for overly extended periods of time. Be careful of any physical damage, particularly puncture, but also crushing. If you ever notice the device is concerningly hot then stop using it for some time to cool down.
Really if you treat it right, then you reduce the risk of something going wrong. If you do ever get a spicy pillow, stop using the device, put it outside ideally protected from the elements while you decide on the next step. Some devices might be beyond repair once it happens so it'll just be a case of disposing of it correctly. Some devices might not be done for but the battery would need to be removed generally you can do it, should you probably not but that's on you. Your local Fire service might be able to help if it's particularly valuable. Everywhere will be different. This does also apply with the batteries before they put on some extra weight and look a little chunky, you should dispose of any old ones correctly. If you put them in your general bin that places you at risk as suddenly it's not protected from something heavy or sharp, or various temperatures or chemicals that could come into contact whilst it's inside your house. Even then after that for refuse collectors, once that bag gets crushed there's a moment where the risk of explosion and a fire is extremely high.
Apologies for the long one but hope it helps. Also folks shouldn't be scared of the technology we ultimately do need it, but being aware of it does help. I think most of all is being prepared for that worst case scenario, and protecting yourself and your family via charging locations and the like. The battery won't kill generally it'll be that people can't escape after it has started.