John,
True for linear/unregulated power supplies, but not SMPSs.
The RF spectrum from SMPS is pretty horrific, the typically loop bandwidth of an SMPS is typically less then 1KHz (due to the phase shift introduced by the output LC filtering).
The typical SMPS LC filtering will be ineffective at RF due to inter-winding capacitance on the LC filters (and the internal parasitics of the decoupling capacitors etc) - it takes a few moments with a simulator to see the effects of only a few pF on RF coupling & conversely RF isolation.
While RF CAN be attenuated, its not in your typical Consumer grade PSU which have little in the way of screening / filter beyond what is required to pass mandatory CE requirements which is more concerned about inducing external interference then absolute levels of external interference rejection.
SMPS cause a HUGE level of RF interference - as a low level signal designer, I'm left to deal with the effects of these on a daily basis...
As with anything, an SMPS can be well filtered / screened, but its very costly by today's "Consumer" standards, and requires skill of the designer which is not understood by your typical (any) China SMPS vendor.
If you where in my shoes, then you would know how many repetitive times I've had to resolve SMPS RF noise interference for 3rd party's and its only getting worst. Sometimes I feel I'm on repeat - some old problems to the "surprised" client.
But that kind of RF enters the system directly, not via the power cord.
RF is injected via any connector port - the mains AC supply is typically one of the bigger problem areas - the proficiency of crude SMPS (and even cruder LED lighting supply's) and Home network over Mains just increases the number of encountered problems in the field.
In fact I have a Patent on a Modulator technique to reduce RF interference - I spent many years researching and developing high performance SMPS / Class D designs - I have a in-depth working experience of there RF performance.
