I just registered here to say I wasn't able to understand why 12VO PSU is going to make any difference in efficiency. I will talk about some sentences from the video with timestamps :
11:22 - "converting from AC to DC power is extremely inefficient and conventional power supplied do it multiple times for the multiple voltages that they need to supply"
Yes it is true and they do it because it is cheaper than having a AC to 12V DC converter and then a bunch of DC-DC converters but as you also said in the video some power supplies actually have that DC-DC converter inside them so this sentence can not be considered as a benefit of 12VO PSU.
11:58 - "to meet upcoming energy efficiency standards (some examples) intel and their partners went a step further that is why the 12V down conversion is taking place here on the motherboard rather than in the power supply because the higher the voltage the less conduction loss we are going to deal with over the runs of wire"
This might actually be true but the difference is just nearly zero, I just used a multi meter to measure the resistance of a 50cm wire from my power supply and it was around 0.002 ohms and it is even funnier to know that the wire loss will actually get higher on full load and in idle it is just under 1 Watt.
13:02 - "So it works"
No it doesn't ! I can easily see that the conventional PSU that you have used is not efficient not because it is not 12VO just because there is no DC-DC converter in it.
So what actually bothers me is the unmentioned cost difference. The cost of that DC-DC conversion circuit on the motherboard is a lot higher than the whole Seasonic PSU you were using. AC to DC conversion is only optimal at a specific current/load and when you replace multiple AC to DC conversions with one AC to DC and then multiple DC to DC ones you get a lot higher efficiency at wider range of load specially idle.
For those who actually care about efficiency, the future is for HVDC power grids. With high voltage direct current, the loss will be minimized in long wires and no low efficiency conversion is needed. Other solutions like 12VO PSU is just for annoyance of customers and nothing else.
And in the end 12VO PSU just reminds me of a laptop power brick, it comes with a single DC voltage isn't it ?