Mainly simplicity; an open-bolt SMG is probably the simplest firearm to manufacture in the world. That also makes maintenance and parts replacement easier.
The normal drawbacks are considered to be:
a lack of accuracy (as the bolt jumps forward, it moves the gun off the point of aim, but at close range, that doesn't really matter)
more safety issues (earlier open-bolt guns were known to be susceptible to accidental firing if the gun was dropped or struck on the butt hard enough to cause the bolt to move to the rear, pick up a round, and then fire it as it's chambered. The UZI was designed with a grip safety to block the movement of the bolt unless that grip safety was held in)
the open bolt itself is more likely to let all sorts of foreign matter into the receiver, unless it's carried with the bolt closed and a loaded mag inserted (which is where the grip safety comes in)
Gun control in Canada: making the streets safer for rapists, muggers, and other violent criminals since 1936.