It can be easy if you know the cause of the problem first hand. Won't boot? Possible issue with one of the components on the power or VRM rails. HDMI won't output? Possibly bad connection with the port or other connecting components.
Unless you know why it's bad, it's kind of hard to determine it just by looking, unless the issue is water-damage or a physical component is missing (resistor, capacitor, etc). Somethings are not visible and you may not find any indication of visible faults. So there might be a short on a board from a faulty component, well then you'll have to locate that short manually and repair accordingly.
Then there are just some things you can't fix, like a broken trace inside the PCB, a bad APU or CPU, or a completely burnt section of the board.
In other words, if you don't have prior experience doing this it's not going to be easy unless you have a general idea of what to look for and the tools to investigate with.