Despite the existence of laws criminalizing holocaust denial and displaying Nazi symbols, Germany still has holocaust deniers and neo-Nazis. Criminalizing speech isn't a very effective way of diminishing it.
Just because a law hasn't eradicated something, doesn't mean that it hasn't been effective in diminishing it. I wouldn't support the United States to follow the same route as Germany, but I understand that Germany was in a very different situation as a country when it enacted those laws. On what basis do you think neo-Nazi activity in Germany would be less diminished today had Germany not criminalized such speech?