The UK's still relatively tame compared to NYC, where I've seen stuff you wouldn't believe (people smoking PCP and blunts, doing backflips and a variety of other acrobatics on moving trains, people talking about murdering people out loud, showing off weapons, rolling joints, showing off bags of coke and crack, bullying the mentally challenged until they cry, Chinese slaves selling batteries, etc).
HK is safe in general, but you might be interested in knowing the HK Police can't really help you if anything happens on the MTR. I know a few people who have been assaulted on the MTR and when they reported it to the cops, they said they couldn't do anything because there aren't any cameras on the trains.