In what way do you feel it's unsafe? There are a few notorious party lines (U8, M10) which can get pretty crowded with tourists on weekends but this is just annoying at worst. There is also a large amount of people begging on PT in Berlin but I never felt threatened by them. I don't take PT much here regardles, usually just ride my bike.
Don't know how unsafe it is, but it is a bit nasty to see the stuff that goes on in U-bahn stations such as Moritzplatz, Kottbusser Tor, Schönleinstraße, Hermannplatz, Leopoldplatz, Boddingstraße et.al.
Never felt unsafe, but the heroin usage and dealing is very open and in your face during the commuting hours.
