Why is a positioned UPDATE statement not completely evaluated remotely?

This happens when the federated database chooses to evaluate the update expression locally before sending the UPDATE statement to the data source. This approach should not significantly affect performance.

Considerations:

Can the SET clause be completely evaluated at the remote data source? In other words, does an update expression contain a function not supported by the remote data source?
Does the SET clause involve a scalar subquery? Does the data source allow this syntax?