>due to the varying thickness of the flange it would be difficult

The person performing the UT needs to map out the joint so that they know what to expect to see while scanning.

Is this channel being welded to the flange of the WF, being used as an outrigger? ie.--> T

If so, this is done a lot and results are usually good if the joint is well prepared, fitted up and welded by a proficient welder.

To answer the question about UT...yes, I'm sure it can be inspected with confidence.