12 volt plug-in PSUs

generally it is fine - apart from one obvious snag and that is the interconnection of positive earth and negative earth items.

These days nearly all electronics will be negative earth, as all 'chip' based electronics is done on  a p type substrate, as NPN transistors are better in terms of speed and gain compared to PNP ones - electrons being faster and 'runnnier' than holes, which are really a clump of electrons going the other way.

The other possible thing might be  conducted EMC issue, where one item modulates the supply voltage drop in a way that another does not like.

Assuming he can use a meter, there is little risk in just giving it a go, but checking things about to be connected are not 12V apart before doing so.


Most things are floating relative to mains earth, so may show 110v AC or so, at a very high impedance, but enough to register on a digital meter and confuse the unwary.


Mike

It depends on whether the devices might interfere with each other, or lead to "earth loops".


Simple example is guitar pedalboard, where the pedals can influence each other, and the DC supply might lead to loops in the ground/shield of the signal wire as it passes through the pedalboard.


"Power supply isolators" are available - I made my own with isolated DC-DC converters, so I only need a single mains input to DC supply, which then passes through the power isolators (DC-DC converters) to power each pedal.

If you are feeding severable items of a larger overall system then ensure all PSU's are fed from the same phase andake sure the negatives are commoned. Other than that I think you're going to be fine. 

regards Ts