SoC is based on one chip that includes (usually) everything like UART, LAN, USB. And sometimes a few chips have to be added to provide more functionality. Added chips makes the boards more power hungry, and you also have to look carefully if there are any sneaky tricks being done that keep the price low, but hurt the performance.

For instance the Raspberry Pi is a wonderful low cost board, with one tiny drawback. The SoC itself does not have a native LAN support. So they decided to to add a low cost LAN chip. Now adding a chip is not automatically a bad thing. But in the case of the Raspberry Pi it is connected to a USB hub. So all the LAN traffic has to go trough the USB HUB and if you want to run an application that if heavy on LAN traffic then the Raspberry Pi might not be the best choice for that particular usage. This is only explain what to look for. The Raspberry Pi is a very powerful little board for what it costs and will be an excellent choice for many applications.