I am looking for specific details as to why isn't GNU/Linux currently SUS(Single UNIX Specification) v3 or even better SUS v4 compliant? What application apis and user utilities does it miss or implement in a non-SUS compliant way?
To get a certification you need to pay, and it's actually really expensive, this is why BSD-like and GNU/Linux operating system vendors like don't apply to it.
So there is even not a reason to check whatever or not GNU/Linux is compliant.
Most of all the GNU/LInux distribution follows the Linux Standard Base, that is free of charge and recognized by almost all the Linux vendors.
EDIT: As my answer is not completely correct, I add the @vonbrand comments
Therefore, does it make it harder to obtain a certification?