You get what you pay for
You can expect that a high-capacity power supply that only costs a couple dozen quid is of pretty poor build quality. Take the Rasurbo Silent & Power 550W for example, it breaks at 300W, has no PEG connectors and a really low efficiency of 70 percent. The three tested Conceptronic power supplies appear to not have any protection against excessive loads and each of them stopped working a 100W below the advertised capacity. Their efficiency was also extremely poor. The Sweex PS060 600W doesn't fare much better, it doesn't even reach its advertised 600W, has a ripple higher than 10 percent and only comes with one PEG connector.
We are impressed that these manufacturers had the courage to submit their products for testing. Consumers, however, should disregard these products.