PS4-Hardware – Which PS4 Special Editions Feature the Updated Power Supply?

As long as the Xbox360 is connected to its power supply the USB ports will have voltage on them. I was able to verify this with a play-and-charge-kit that showed me the LED colour for "recharging", when my Xbox360 was turned off.
I cannot verify if the USB port will supply enough power in this state to actually charge a battery.

The power that is supplied by the USB ports in the back will surely be the same as for the front ones. USB specifications state that any high power port has to be able to deliver 5 unit loads of power, which is 500mA. Seeing as the back USB port is intended for a Wireless LAN USB connector, which usually does not have its own power supply, it has to be a high power port.
Wikipedia USB Power
USB technical documentation

What devices are susceptible for hardware damage by a sudden loss of power?

Computer systems and other electronic devices containing logic circuitry are susceptible to data loss or hardware damage that can be caused by the sudden loss of power. These can include data networking equipment, video projectors, alarm systems as well as computers.

The electricity flowing through your home is not constant. Ideally it would be, but the reality is that electrical currents can ebb and flow, sometimes dropping in voltage and other times surging with extra power. All of these can have undesirable effects.

When power completely shuts off, it’s known as a blackout. These tend to occur due to issues beyond your control (e.g., power station disruptions, damaged electrical lines, etc.) but sometimes they can be self-inflicted (e.g., by shorting or overloading circuits).

And then there’s a similar issue called brownout, which is when your electrical voltage experiences a temporary drop without fully blacking out. If you’ve ever seen your lights dim for unknown reasons, it was probably due to a brownout. Brownouts can be intentional as a way to reduce electrical loads and prevent blackouts, though they can be unintentional as well.

The real danger of blackouts and brownouts is the unexpected computer shutdown. Operating systems are complex and they must go through a “shutdown sequence” to make sure all running processes have correctly terminated before powering off. A sudden loss of electricity can interrupt important threads and leave your computer in an inoperable state.

System files are the largest concern. Consider what happens when a hard drive is writing data to the disk but suddenly shuts down in the middle of it. Suppose the file being written was a system file necessary for the booting process? Now that file is corrupted and you can’t boot up your computer without going through an involved recovery procedure. Furthermore, frequent power outages can reduce a hard drive’s physical lifespan. The read-and-write head, which hovers over the spinning platters during operation, snaps back into its original position upon power loss. This sudden movement can cause tiny imperfections that accumulate over time, increasing the likelihood of a “head crash”: a malfunction that occurs when the head touches and scrapes the platter surfaces, effectively destroying the hard drive.

Solid-state drives can also suffer catastrophic damage from sudden power cuts. Issues can range anywhere from data corruption to total malfunction. It should be noted that although power outages will not cause direct harm to computer hardware other than data drives, it’s possible for power outages to be accompanied by power surges, which can cause severe damage to hardware.

TLDR;

To sum up, on a power failure/sudden shutdown/blackout your hardware can get corrupted files. This is due to the fact that the hardware, when shutting down, needs to go through its 'shutdown sequence'. If your hardware is writing an important file and it suddenly loses power the file that it was writing at the time became corrupt and is most likely deemed useless. (There are ways to fix this but thats not the question here). If you in your case didnt experience any downsides (corrupted files/hardware failure/etc) you either were very lucky or your system repaired itself by using files that weren't corrupted.