I'm not sure on the AdUnk %, but it was probably more like 50%. They did every so often do a mailing with address correction requested. And yes the BMO stat going down was not a happy event, but you can take the hit one week and then carry on.And I'm going to guess it was at least 90% "Address No Good". Or closer to 99%?
People move, and orgs were typically reluctant to spend extra for "address correction" mailings (where the post office sends back the forwarding address of people who move, which is only kept for about 6 months). After a while, your mailing list is mostly rotten.
Another reason why org management was reluctant to do address correction, was that it would then show up the addresses which were no good, and thus needed to be removed from Addresso. Removing addresses from the list meant that the Bulk Mail stat went down.