>heresy
Well it all depends, doesn't it?
Since heresy is defined as going against a doctrine of a given denomination, whether it is heretical depends on what denomination you are.
All of the heresies that "The Shack" is accused of are perfectly valid in at least one Christian denomination.
Universal reconciliation is a valid belief for modern Christian Universalists.
Denying free will is a valid belief for Calvinists.
Sola Fide is a valid belief for many Protestant denominations.
Many denomination reject the Penal Substitution theory of Atonement
The Shack has all these "heresies", but no one denomonation believes all these things. Which is why a Christian from any given denomination is going to find something wrong with the book.