In theory, I'd think, a confederacy would be more unstable than Civ's "democracy," because of the right of secession. For that reason, I'd give it high war weariness.
I'd also give it high stats for economic growth, because of regulatory competition between member states. The labor and business communities are free to move to the friendliest zone the confederacy has to offer. In other words, minimal corruption and use the trade bonus.
I'd also guess that assimilation would be low or none, because a weak central government could not exert much pressure on its members to conform to its standards.
I agree with forced labor (in the above post) only if you are mimicking the Confederate States of America. Otherwise, it's a non-issue. In fact, I'd say an intellectually honest confederacy would not have forced labor.
You could give a confederacy some free units, too, because I'd think that member states would shoulder some of the financial burden, and also promote local militia systems.