Members of the ATF have the power to break up criminal organizations, stop terrorism groups, investigate acts of arson, prevent illegal firearms sales, and protect American citizens. However, certain laws often restrict the ATF's power to easily conduct investigations.
For example, in certain states, legitimate gun dealers like Don's Friendly Assault Weapons don't have to report anyone buying a large cache of assault rifles (source). The law only requires that gun dealers report the sale of more than one handgun. Yikes.
Furthermore, the ATF is only allowed to review a gun dealer's records once every year, and even then often lack the manpower to follow through (source). That lack of reporting can make it easy for criminals to simply swing by and load up the truck with AK-47s before hitting McDonald's for their morning hash browns. Many efforts to strengthen such gun laws have historically petered out.
While fights over gun laws are still going on in Congress, the ATF is trying to make do with the amount of power that they have. That being said, the ATF isn't immune to abuses of power. Like many other law enforcement agencies, the ATF hasn't been immune to scandal, including accusations of searching people's property without a warrant, falsifying documents, and giving inaccurate testimonies. Even the ATF has to deal with some rotten apples.