Musical America just released a PDF titled "Securing U.S. Visas for Visiting Artists". I highly recommend it. Interesting to see just how complex the system there has become.
The most relevant story for me, was that of the Chicago Opera Theater on page 17. Here the up-and-coming Irish singer Claudia Boyle was to take the role of the Queen of the Night in a production of The Magic Flute. Unfortunately, the process dragged on so long that she ended up missing the show. The U.S. Consulate in Dublin "was inclined to send her application back to the USCIS and request revocation because the reviews weren't good enough, the awards not international enough, and the artist was too provincial."
What a Debbie Downer.