The issue with vunit+ghdl resides in that the GRLIB does not fully compile with the standard 2008. It does with the standard 2002 though. GHDL uses by default what they call group 93, and it includes the 2002. If the GRLIB is simulated with ghdl, it runs without errors. However, VUNIT uses the standard 2008. It provides the option to pass the standard to libraries and files with add_source_file() and add_library(). This option works well in Modelsim. However, ghdl cannot mix different language standards. Although, I would like to see the GRLIB fully compatible with the std 2008, the work around is to avoid some files the gaisler/stdlib and a few debug only functions in some files i.e. ahbctrl.vhd, basically the ones which prints the information in the Amba bus.
I hope we will have a GRLIB fully compatible with the std 2008, not even mention the std 2019