Partial Mocks are bad, why exactly?

Partial Mocks are bad, why exactly?

Scenario
I have a class call it Model that represents a complex, composite object
of many other objects of varying types and purposes.
I have a Metrics class which calculates some more or less complex metrics
about the Model, in essence it looks something like this:
public class Metrics{
private final Model model;
public Metrics(Model aModel){model = aModel;}
public double calculateSimpleMetric1(){...}
public double calculateSimpleMetric2(){...}
public double calculateSimpleMetricN(){...}
public double calculateComplexMetric(){
/* Function that uses calls to multiple calculateSimpleMetricX to
calculate a more complex metric. */
}
}
I have already written tests for the calculateSimpleMetricX functions and
each of them require non-trivial, but manageable amounts (10-20 lines) of
setup code to properly mock the related parts of the Model.
Problem
Due to the unavoidable complexity of the Model class stubbing/mocking all
the dependencies for calculateComplexMetric() would create a very large
and difficult to maintain test (over 100 lines of setup code to test a
reasonable representative scenario, and I need to test quite a few
scenarios).
My thought is to create a partial mock of Model and stub the relevant
calculateSimpleMetricX() functions to reduce the setup code to a
manageable 10 or so lines of code. As these functions are already tested
separately.
However, Mockito documentation states that partial mocks are a code smell.
Question
Personally I would cut the corner here and just partial mock the Metrics
class. But I'm interested in knowing what is the "purist" way to structure
this code and unit test?