Currently custom multi-select fields are not queryable and cannot be included in reports. We have a custom multi-select field that identifies the type of work product being reviewed, which means that our users cannot use Collaborator's reporting feature to get a list of all reviews for a particular work product, and also our users cannot see the work product from the report summary page.
If Collaborator could be enhanced to include this feature, the built-in reporting feature would be much more usable.