In 1.6, the TabletBalancer APIs used org.apache.accumulo.core.data.KeyExtent. In 1.7, this class was deprecated in favor of TabletId. Unfortunately with this change, the TabletBalancer API was updated to use org.apache.accumulo.core.data.impl.KeyExtent. This causes compile time issues with balancers from 1.6.
I'm not sure if the TabletBalancer is technically part of the public API, but given this is a user pluggable space I would expect some degree of stability, at least between two versions.