J. Edgar Hoover probably crossed a lot of ethical lines in his tenure in charge of the FBI, if not legal ones. He was not afraid to wield his law enforcement power. While it is regrettable that anyone innocent may have suffered persecution, the guilty and criminals definitely also did. Hoover's methods and use of the FBI are not exactly something to be admired, but they did produce results which left the nation safer.
Yes, J. Edgar Hoover's founding of the FBI made America a safer place. By creating a unified organization to handle criminal and terrorism matters on a national, federal level, the country benefited by having a cohesive group to monitor potentially dangerous on goings. Without an investigative unit on a national level, such as with the FBI, the country's investigative units would have been highly fragmented and disorganized.
Hoover went far too far in his pursuit of justice as he saw it, and he did not, in the long term, make it safer. Hoover's dealings with the mafia made him ignore them and allow them to gain power and organization, and he blackmailed political leaders into getting what he wanted.
I do not believe J. Edgar Hoover made America safer as the FBI's first director. His time with the Justice Department and the FBI was during the Red Scare when the American government literally went out and harassed anyone they deemed to be a radical. This is well documented by the entertainment community at the time because they were singled out as part of the problem.
Law enforcement as whole may make the contry safer but I do not believe the FBI, now or under J Edgar Hoover, significantly makes the country as a whole safer. The one thing that J. Edgar Hoover's FBI was involved in that may of posed a threat to the country as a whole was communism. I do not believe though, that it actually amounted to that much of threat on the countries own soil.