For example, you may find there is no place to mount a second disk drive, and no power cord for it. This is a good argument for assembling a computer from parts you choose, with guidance from someone who has assembled them before.
Are power supplies in newer computers coming with lesser power supplies that there are not enough pigtails for extra drives?
I also didn't get into USB connectivity. Any recent computer case will have several ports, and you can buy inexpensive hubs to augment them.
Wall powered. Trust me. 10 port hub that holds 10 devices is under $50 and it doesn't draw power from the computer power supply. And data flow is not diminished at all. I used to have a 7 port passive hub and all my ports were in use. Now 1 port is used (for the hub) and everything is powered from the wall.