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. That's why BSD-like and GNU/Linux OS vendors don't apply to it.
So there isn't even a reason to check whether GNU/Linux is compliant or not.
Most of all, the GNU/Linux distribution follows the Linux Standard Base, which is free of charge and recognized by almost all Linux vendors.
... therefore, does it make it harder to obtain a certification?