The ERC check in kicad is quite basic. It only checks if pins of one type are connected to their correct counter parts. The checking matrix is in the second tab of the ERC dialog.
As the ERC depends on the correct setup of pin types, it is important do understand the options you have in kicad. See: Electrical type of schematic symbol pins (KiCad 4 and KiCad 5)
If you get lots of pin not connected errors for pins that are intentionally left unconnected use the not connected “Flag” (small blue x) to tell ERC that this pin is indeed unconnected on purpose.