Hi T
It is not by default a bad decision to install GRC on the ECC environment. However it is not a decision basis should make.
To an extent I agree with you - the decision should be about reviewing options and making a risk-based decision that the business can accept.
However, if a vendor-owned installation guide puts a warning/recommendation not to do this would you take that risk? If you raise a customer incident your issue may not be resolved if the vendor's recommendation conflicts with your ECC functionality.
I've already responded below - hearing the project is purely IT driven is going to cause issues beyond which system to use.
Regards
Colleen