I understand the benefit of single rail.
Multi rail does not mean junk. Antec HCP platinum units, the fully modular Enermax and Lepa, and the bequiet platinums are three good examples of excellent multi rail designs.
I prefer multi rail, on a large PSU, for the obvious reason it's safer. If everything is running off of a single 100A rail and there's a malfunction it could be catastrophic for your whole system.
Here's a quote from the AX1500i review on JonnyGuru
I can understand that a lot of people don't know, or can't be bothered with load balancing. also, with good PSU's like MrTeal's it's not a likely occurrence to have issues. I'd just rather be safer.
The thought that multi rail is junk though is just misinformed.
Well, I guess there's a bit of a misnomer here. The AX1500i is a single rail design, as are the vast majority of large units. There is a difference between a multi-rail design and one that has OCP on the outputs to prevent cables melting. In a traditional multi rail design, the power partitioned into rails wasn't available on the other rails. For example, I had a 1010W GamerXStream years ago with 20A/20A/20A/20A on its four 12V rails. 12V1 was for the CPU, 12V2 was for the MB and peripheral connectors, and 12V3 and 12V4 were for PCIe. Even with perfect load balancing on the PCIe connectors, you are limited to a total of 480W to those PCIe plugs. If you didn't need 20A for the CPU (most don't) and 20A for the MB (who does?) you'd need to use dual Molex to PCIe or EPS to PCIe cable in order to tap into that.
OTOH, dividing up a single large 12V rail into multiple smaller "rails" still makes it a single rail design. There could be no rail markings (as it is on my X1000) but still with OCP, or they could divide it into groups where you might have OCP on one set of plugs and call it a rail. That's the way the Antec HCP1000 works; the 12V outputs are collected into groups of two or three cables, and a 40A limit is applied to that. You never really lose capacity though, since you can pull the full power from any group as long as you don't exceed ~500W per "rail". I'd argue the Antec method is actually less safe than the per-plug method that Seasonic uses, but it's really six of one and a half dozen of another.
Edit: There are of course single rail designs with no OCP which can be a safety issue, but there are also (shockingly) units marketed as multi-rail with limits per rail that don't actually feature any OCP either.