I had a bathroom remodel done. So, I got a higher CFM fan. Of course the builder put in the cheapest ones made. The exit piping was larger on the new fan, I think it went from 4 inch to six inch. It was vented through the soffit, the unit came with an adapter to reduce from 6 to 4. Didn't make sense to reduce it and restrict the flow. Got some basic duct work and ran it up to the vent ridge. I would not suggest venting directly into the attic. Might be fine, but if it isn't you are gonna have quite a mess to fix over time. Moisture is always a problem you want to prevent not fix.