This package provides the Xilinx FPGA Unisim library as a set of VHDL snippets.
Xilinx FPGA Unsim libraries are or can be used for instantiation of hardware dedicated logic elements available in the different FPGA families, elements such as PLL, MMCM , LUT, ...
The list is not a complete copy of the Unisim library available in the Xilinx Vivado tool! It does not contain elements as PCIE or Ethernet MAC because those elements are too big to instantiate and one will probably never do this anyway because these components are typically used through a setup wizard. The wizard provides an instantiable model targeted and customized following what the user demanded through the wizard.
Good catch. Let us know what about this package looks wrong to you, and we'll investigate right away.