A longer cable won't just catch fire, because the power dissipation per unit of length is the same regardless of overall length. Imagine the most extreme case - a cable so long the voltage difference is 0V at the end. It's basically just a very long resistor dissipating 60W. But each meter of cable will be dissipating the same power as every other PoE setup.

Looping the cable or putting it in a confined space could cause issues. The cable could then catch fire even though it appeared to be operating normally to the PoE controller.

In my understanding, it depends on the device and the cable installation. Let's say it's a 59W device so it doesn't fall under NEC regulations as of 2017 for PoE devices over 60W.

The device needs a certain amount of power to keep itself alive. Depending on how the device is designed and if actually adhering to standards, the device should simply not have enough power to start at say 80m, or let's say they pushed the install from the get-go (happens all the time) and it's actually 110m on poor / underspec'd cable.

And let's say the device has enough power to start, but you're using indoor cat5 and it's been outdoor for 7 years, and you don't know this but it's CCA. If it's in a bundle with other similar devices drawing high power and there is enough heat concentrated at a bend, then yes, the cable could catch fire without the device having a problem. As long as the device has enough power it's going to keep doing its thing until the cable has degraded enough to cause signal drop and assuming it's using one of the more modern 4pair PoE standards, would just shut off. But that could be after the drapes or that amazon box in the corner of the room caught fire.

We're just lucky in the residential space that PoE hasn't been as "mass market" as an iphone, and we've been slowly working into higher power delivery as demands have increased.

IMO? It's all silly though. We should just go optical and direct-DC whenever possible ;)

Depending on switch vendor and quality, they can actually increase the voltage output - the spec iirc allows for up to 57V at PSE which an intelligent switch can modulate to overcome limited voltage drop - cheaper switches (desktop etc) just supply all ports 54V (or less, but it should be 54 at source) from the same rail without any modulation

The specs have a voltage range that the source can put out and a corresponding range of voltages that the device must accept at the end of the wire, after voltage drop.

Longer wires don’t increase the overheating risk because the additional heat is divided over the additional length.