Who'd Want To Be A Librettist?

Charles "two minds" Spencer gets confused at Into the Woods:

"With characteristic ingenuity, Sondheim weaves together several fairy stories..."

But (there's always a but):

"But in the second half the musical falls apart. James Lapine’s book becomes an increasingly confused mess of plottage..."

So let me understand this. It's the songwriter, Stephen Sondheim, who weaves all the stories together so ingenuously. But it's the book writer, James Lapine, who messes up the plotting in Act II.

Ah, book writers: none of the praise, all of the blame.