Simple answer is: A welder qualified to Section IX does not meet the qualification requirements of AWS D1.1. Why?
1) The visual acceptance criteria of Section IX is very sparse compared to D1.1. ASME Section IX does not include acceptance criteria for:
undercut
overlap
porosity
face reinforcement
2) The acceptance criteria for guided bend test results of Section IX is not as stringent as AWS D1.1.
That's my story and I'm sticking to it. That being said, there are provisions in D1.1 for the Engineer (Owner's representative) to accept qualifications to other standards.
Al
Best regards - Al