|The logic behind multiple rails was to feed clean power to each of TWO or THREE graphics cards ( SLI & Crossfire) plus feed the CPU and then have more rails left over to use for drives, etc.|
The problem with most multi-rail PSUs is that while there MAY be enough power overall it may not be enough on each individual rail.
For example, a system with a single high performance graphics card may need 25Amps just for the card. The individual rails may not have 25A capacity.
On single large capacity rail doesn't necessitate balancing the loads either.
The reason many PSUs with multi-rails are on sale is because manufacturers are reverting back to single rail configurations.
Going back to the multi-rail setup, one rail is usually dedicated to each graphics card and will have the special PCI E power connectors. One rail will be to feed the CPU directly and will have the special connector for that.
Hope this is clear enough for you.
So, some rails may not have enough power and some too much.