In my mind the one that could give best answer is probably Bobcat but he is for some time not around anymore.
Not trying to argue, the issue with multi rail is that there is more parts needed and more possible wastage in term a device such as a GPU need more than a single rail at once, making it harder to achieve same efficiency compared to single rail. Most multi rail use the same power source (first stage when power comes from the wall) and are splitting it at some point instead of having it splitted from the very first stage already. So there is different setups and different complexitys, but most of them simply add splitting at some later stages for security purposes and nothing more and may have rather minor splitting techs. In term GPU is using 1 rails and MB 2 rails, and lets say the GPU is lacking energy, then the energy cant be gotten from the MBs rail, so there is in fact much lesser power available to the parts because of strict ressource handling. But in the end most multi rail work in a way that is just adding a security layer but not using a genuine rail at every single stage so if stage 1 (the stage with the biggest capacitor) will break every rail will become troubles. In that term more a hassle than gain and multi rail is becoming less popular in nowadays high power PSUs. Finally a genuine (most are fakes, just added at some late stages) multi rail is much more complex and pricy, so most manufacturers are avoiding it. In the end, single rails overload protection will only kick in at very high Ampere ranges (~100A when 1200W+ PSU) so the possible damage is higher, although parts can be damaged even at low Ampere (40 A for example, some may consider it the "safe max value for single rail"). It is more important to have good protection mechanisms and nowadays the advanced supplys are more mature and can detect a short circuit so fast that it wont truly make a difference, 100A or 40A, it will instantly shut down with almost same risk. On the other hand, when a multi rail is with bad protection the 40A or whatever limit wont safe the parts, no matter how many rails. I already had a GPU blowed up and my Corsair 1200W supply was kicking in so quick that i had no damage to other parts at all... although over 100A on a single rail possible. It seems, nowadays even the high power PSUs from big names (Corsair, Seasonic) are becoming more and more single rail spec because the advanced protection mechanisms are sufficient even to be secure at high Ampere and it will cut the cost, possible complexity and OCer can freely use the Ampere without any limitations. The even more advanced supplys may even have a Ampere "adjustment", so it can be artificially limited, so the "hard coded" PSUs could soon be a "stone age spec", technology is more advanced nowadays and rails is with even lesser meaning. The rails at some future advanced digital PSUs that have a separate port management (i bet Corsair gonna do the lead) are even able to offer some rail management (adding, combining or removing rails at certain ports) for the late stages of the PSU which is sufficient for protection, although not a genuine first stage rail and such a spec is pretty much deprecated with todays technology.
