[edited to ask SkyHawk what there was on the course that put him off composite?]
The NDT part of the course gave us hands-on experience on the following techniques over several weeks;
Eddy currents
Magnetic particle testing
X-ray radiography
Infrared thermography
Liquid penetrant inspection
Ultrasonic testing
Acoustic emission
Not many are useful when inspecting for impact damage and delamination of composite materials, due to the very nature of their structure. Ultrasonic is by far the most reliable but not infalible. The FAA recommend the coin tap test, basically dropping a dime on the material and listen out for a difference in the sound, not too techical but suprisingly accurate. NDT of composites is still in it's infancy and there are other techniques we didn't look at in very close details, such as Laser Shearography but I don't believe these are in wide general use at the moment.
NDT of metal including aluminium is a proven science and many of the above methods come up with reliable results. Even an amateur like myself can detect subsurface cracks and flaws as well as the easier surface faults with reasonable accuracy, although obviously a more experienced NDT techician achieves a much greater accuracy.
Composite damage is cut out and filled with replacement material which may or may not be bonded correctly to the existing structure and then covered over with resin and filler making the repair no longer visible. It's difficult to detect whether the repair was done using the correct temperatures and timings, etc, important for the correct strength to be achieved.
The majority of repairs to aluminium are more easily inspected. Rivets can be counted and measured, and sizes and thicknesses of patches can be measured. As as I said above, metal integrity can be relatively easily tested using widely accessible NDT devices.
In summary, I personally believe that damage is generally easier to detect on aluminium aircraft and repairs generally easier to inspect due to the repair techniques used on aircraft. I imagine that a less professional composite repairer can get away with making very unsatisfactory repairs and still make them look good.
If there are any professional NDT people out there it would be interesting to hear what you think.