A ProjectVelocity metric.
The NebulousUnitOfTime metric still indicates the estimate has something to do with time.
Gummi Bears, by contrast, are sticky. They clump together. A sticky intractable problem might not take much time, but it might still wear down a team's "hit points". A given hard problem might require brief pairing between only the two most qualified, followed by everyone else reviewing their solution.
See ActualGummiBearsForEstimation and GummiBearsConsideredHarmful.