I'm fairly certain that the problem with many of those older multi-voltage supplies is that they used the 5V supply as a reference for the 12V supply, and that the 5V supply was of a cheaper-to-make design that didn't regulate well unless loaded to 10 or 20 percent of its rating.
I think the reason newer PC supplies do better is that they have to be designed to work in "sleep/standby" mode, and keep the 3.3V and 5V supplies under control even when very lightly loaded. As a result, they can't use the older circuits.