You might be able to run on a 5W supply, but my experience is that booting on a less than 15W supply is a recipe for corrupted SD cards and hard-to-trace bugs.

My biggest frustration with hobbyist boards is how little power the can source to e.g. the USB ports. Its super frustrating to need a powered USB hub to add e.g. a USB indicator light. The fact that you can power the RPi from the powered USB hub seems hacky, but maybe that was always the intent.

Try to run even a Pi 3 on 5W and let me know how it goes. You'll get undervolt warnings like crazy. Not sure if you looked at the article but these are the power adapters listed. These SBCs are not exactly low power anymore:

5V/5A (USB-C) 5V/4A (USB-C) 5V/5A (USB-C)

You'd need to limit the CPU operating points or offline most of the CPU cores to get lower max power consumption under load. 5W is too strict for Orange Pi 5 Plus. And of course forget nvme if you want predictable low power consumption. nvme alone is 5-10W load. The board itself can run under 5W, but you'd have to carefully consider what peripherals you can use and how.

See https://xnux.eu/log/088.html

M600 idles around 5-8 watts. If you spank it hard it gets up to 12W. That has a 500Gb SATA SSD and 8Gb of RAM and ethernet.

Note that the M600 does not have any active cooling. The entire unit is a big heatsink. The thing will run as a headless network computer with just the power supply (a proper Lenovo SMPS brick) and an ethernet port connected.

power usage on a computer is not constant by any measure.

power usage spikes with CPU activity (among many other things) and if your power supply can't deliver the current that is required, voltage will drop, and things start failing, but only for 1ms or maybe even less. maybe even a single clock cycle in some cases.

if you can't keep the board powered fully for every clock cycle, then you are going to have problems, and that's true for any computer, not just the Pi.