TezosKit provides advanced functionality and extensibility out of the box.
Users can create their own RPCs and Operations to support operations on the chain which TezosKit does not provide support for.
See:
Every operation in TezosKit is pre-applied before it is injected in the block chain to catch errors.
A future update of TezosKit will make this functionality optional.
Forging of operations is currently done remotely.
A future update of TezosKit will make this functionality available locally and provide a toggle to change functionality.
Users can set custom fees on operations. See the Fees Documentation.
A future update of TezosKit will provide gas estimation.