I'd probably like it better if the a capella part was maybe four hours shorter.
I think I know which song you're talking about, and I agree.
I personally liked Hallowed be Thy Name and Aces High Live, studio would have been better but those to along with Fear of the Dark obviously were still great in Rock Band. Iron Maiden live of the other hand was meh and it would have been so much better if we had just gotten the studio version of that song. Most Live songs on Rock Band have been absolutely spectacular, such as the Tom Petty, Kansas, Thin Lizzy, AC/DC and others.