Karma is a forum field, not in CB tables, and where the result can be called by CB in the forum component, thanks to the corresponding CB plugin. The current CB 1.1 API design that we are using there doesn't allow us to know the corresponding database structure used by the particular forum component, and how to link that to CB database by SQL formula.
Therefore a full search would be very slow and extremely database taxing, as not done in SQL.
That's why the "external fields", like karma, total forum posts, etc can be displayed for each user shown in the list, but not searched, for now.