Bad - not only the balls hitting it.... I see heat being a major downside.
Heat isn't a concern according to the article:
Roy insists, though, that sufficient precautions have been taken to ensure the safety of the fish. For instance, the tanks are carefully temperature-controlled, ...
It's a guess as to how well sound travels through really, really thick glass, if that is indeed what it is.
I think it's a cool idea and not nearly as bad as many are making it out to be.
The one thing I think might have been overlooked - cleaning off the algae that will grow!