Loading one of these supplies up to about 80 - 85% of its Peak load specification and have a look at the output with a 'scope. With AC coupling in the vertical channel, you should see ripple and noise that is less than 100 mV. Anything greater, including momentary spikes, will be detrimental.
While you may not hear anything objectionable in the speaker, it is quite possible that it will affect the transmitted signal, or the decoding of a digital signal such as PSK, JT65 etc.
As far as I am concerned, for the value, a linear type of supply, while not being as efficient as a SMPS, is a much better value in the ham shack.
These are my summarized thoughts on this power supply. The positives are that it's small and light weight for a supply that can deliver 75 amps of current. I REALLY like all of the wire terminal choices and I also like that they are on the front. If you don't play with a lot of different types of equipment, disconnecting and reconnecting, you would probably prefer that the terminals were on the back. Variable voltage and meters are a nice touch, and I LOVE the battery charging feature. The negatives are that it is noisy (this is not necessarily unique to this switching supply) and that it probably could have better RFI filtering designed in. While I haven't personally experienced any problems with RF hash, I will be keeping a close eye on it.
In my mind, this is one of those types of products that are "good enough" for what I need in the ham shack. As long as I don't start to experience RF hash, I will be happy with it as it gives me much more room for other radio equipment
After owning this supply for more than 6 months and using it regularly, I can safely report that this supply DOES put out quite a bit of hash that affects the HF bands. It exhibits itself in the form of something that sounds like a moving carrier (I might try to shoot a quick video so readers of this post can actually hear for themselves). It is also visible in an attached panadapter.
I've been doing quite a bit of research on RF hash caused by switching power supplies, and while the problem is not as bad with lab-grade, expensive supplies, those are probably out of the reach financially from most hams. I have found a fairly decent solution to the problem with this MFJ supply that is fairly affordable - I will post details soon.