The main disadvantage of an open bar is that guests are able to order drinks without any supervision. This sometimes results in unruly or loud behavior that can be disruptive during a formal event. Also, if the bar has seating, it has the potential to draw attention away from the main attraction if a group of friends suddenly sits down.
Cost can also quickly become a concern with this type of arrangement, since overindulgent guests might consume far more liquor than was originally planned. If a restaurant provides the bar, then actual receipts could determine the cost of the bill at the end of the night. Open bars are almost always more expensive than other options such as cash bars. The cost can increase if the host allows people to order expensive types of liquor. Some restaurants and catering companies do not stock expensive liquors while running this type of service unless specifically requested by the host.