Jonas Kaufmann has spoken up more than most. Here is what he had to say to the NYT in 2012 after a Faust premiere:
 

“What did he think of the new Met production of “Faust” in which he had just sung? No comment.”

“I am too much of a diplomat,” he said. “But I will generalize this much. Too many directors arrive at the opera house these days knowing little or nothing about music. Most come from the spoken theater, focus only on the text and don’t understand how to give the music its space. It may seem obvious to you and me, but a brilliant theater director does not automatically translate into a brilliant opera director. If I am a crack racecar driver, that doesn’t qualify me to be an ace pilot as well.

“I sometimes feel that directors devise all these elaborate concepts because they don’t trust the power of the music and are terrified of boring the audience. Opera is a truly magical art, but the magic originates primarily in the music that we singers work so hard to communicate.”

https://www.nytimes.com/2012/04/22/arts/music/jonas-kaufmann-as-siegmund-in-die-walkure.html