(I'm not familiar with the iso, but for me its the PCN. its more challening than the EN4179 but its a perrsonnal certificate. The EN is an employee based certification which means the requirments are set in the practice manual and might not include all the aspects of the method.
As an example in eddy current your certification might include high frequency, bolt hole and conductivity but without low frequency simply because the company practice manual does not include it or its not required in their maintenance
After all that is my perosnal opnion)