Competency frameworks fail for the opposite reason performance systems do: they are usually too sophisticated, not too crude.
Describe behaviour, not traits
"Strategic thinking" cannot be assessed consistently. "Identifies the second-order consequences of a decision before recommending it" can.
Keep the scale short
Four proficiency levels, each with an observable description, will be applied consistently. Seven levels will not.
End with a development plan
An assessment that does not produce an action is an audit. The output of competency mapping should be a short, specific list of what each person does next.