When pre-release macOS Sierra is in safe mode with
-x as a boot argument in NVRAM, the following command works as expected:
sudo nvram boot-args="-v"
When Sierra is booted in normal mode, the following command is rejected by the OS:
sudo nvram boot-args="-x -v"
- is that rejection to be expected with SIP?
As a workaround, I typically use Recovery OS Terminal to run the nvram command.