I did some trials on this about 4 years ago, Usuing a MHz, 24 Element transducer
1) The 'noise image' from a good weld was clearly distinguishable
2) Side drilled holes around 3mm gacve a good response.
So it all looked very encouraging. Unfortunately we never persuaded any of the Railways to buy one, So it never got seriously field tried, and there was not opportunity to find defects with the technique.