We returned a car to the Bologna airport in September. I'd read somewhere that Bologna airport taxis will only take you to the Centrale train station if going to the city center, but we just grabbed a taxi from the stand at the airport and no issue to be dropped off in front of our apartment which was several blocks from Piazza Maggiori. For two people the fare was somewhat less expensive than the Marconi Express.
When we picked up the rental two weeks earlier in Treviso, Sixt gave us a ticket to get through a security gate to enter the car return area in Bologna. But we didn't realize the ticket expired before our rental end date, so we couldn't enter the gate and it became a real hassle to return the car. I'd return a car to the Bologna airport again, but check that security gate ticket expiry! The rental car return area at the airport is a two-level parking garage shared by many (all?) rental agencies, a short, level walk to the taxi stand.