Things haven't really changed. Typical computer switching supplies still need a load on the 5 V and often the 3.3 Volt bus to startup, and a connection to simulate the "power up" signal from a motherboard to start as well. Many of the newer supplies actually have lower 12 Volt capability than in the past.
Originally Posted by KF7QLP
The problems are multiple:
1. The "12 Volt" rail is usually +/- 10%, which is sufficient for hard drives and DC/DVD Drives in a computer, but many Amateur Radios will not work well, (some won't work at all) if the voltage falls below 12 Volts. They ARE actually designed for 13.8 Volts, +/- 10%, since 13.8V is the typical automobile system voltage when the engine is running.
2. The (RF) noise generated by such supplies can be horrendous; most of the supplies do NOT have any RF suppression or power-line filtering. (Open one up. You will see where the filter components are SUPPOSED to be mounted, but those spots are only provided when the unit is first submitted for RFI testing; once in production, the filter components are "conveniently" eliminated.
YMMV. You MAY be able to use such a supply, or you may be bitterly disappointed. And different brand supplies can vary widely; even different models from the same brand name may have widely different performance.