Well as far as CPUs are concerned, one thing that was a big issue for me was the manufacturing plant...
With an OEM CPU, you get what you're given.
If you have the luxury of going through a few retail boxes, you can select the mfg plant, and mfg week of your choice.
It's this kind of difference that's usually more appreciated with Intel CPUs - it was how I assured my c300a could do 540,
and no doubt helped my c850 do 1.14, with a WBK68, never going more than 3 degrees above ambient temp...