Comparing between fields is a surprising implementation of the Compare condition. Most implementation use a ConstantExpression or a QueryExpression for the right expression (available when the field on the condition is a reference).
Comparing between fields seems to be working, but I can't say it's officially supported and recommended. You may want to choose a more conservative implementation, i.e. reading all relevant entities from Clarizen and filtering the results.
Having said that, Duration fields have a numeric value in working days (as per the default hours per work day in the organization calendar). I managed to implement such Compare condition between a Duration & numeric fields.
If your numeric field's value is not in working days, I'd suggest you use Customization to setup a formula field that does the 'unit' conversion, or maybe better still check if FieldA is greater than FieldB leaving more standard conditions work for the API.