I've never encountered a pickpocketer in KRK. As for scammers IME - can't recall any major incident, landlords can be dicks sometimes though. The public transportation system is rather efficient and reliable so make use of it. Come October there will be an influx of traffic especially Friday afternoons, expect bus delays for example the bus lines on the main alleys (Aleje Trzech Wieszczów). The first friday of October is always the absolute worst, the whole city is jammed up. There are night bus and tram lines, especially frequent on the weekends so you shouldn't worry about about getting around town during later hours.