FWIW, our (German) company has had a travel warning for the Hauptbahnhof area for more than a year now and we are explicitly advised not to book hotels there by corporate security. Sure, the majority of junkies might be harmless, but as others have pointed out, drug crime and prostitution go hand in hand with gang activity and violent crime, and there has not been an exception here either. Will you most likely be fine staying near the Hbf.? Sure. Are there safer, more pleasant places to stay? Definitely.