Standard unimer have low sampling speed or bad display refresh time, but if PSU have very bad ripple you can see flickering led 7-seg display digits of your project, or such things. My opinion that is best way in that situation to use 12V from PSU and voltage regulators for 5V and 3,3V with appropriate capacitors in input and output stages, and of course standard usage of 100nF closer as possible to VDD/VSS pins as usually.
:wink:
I use very often PC PSU when working on various projects.