Give over you can visit the main drinking strip of any similiarly sized city and take photos of drunken idiots especially when you've set out specifically (as this photographer has done) to find them.
I've lived in cardiff on and off for ten of the last 15 years and i've never had a single moment of trouble in st mary street let alone any other part of town. Unless you go looking for it (or are a welsh rugby union international - google it) you'll struggle to find any trouble. Seriously if anyone is that concerned you should think of never leaving your house ever again.
Cardiff has the usual mixture you'll find in any medium sized city. If you find the city centre too boisterous you can always try the more genteel cardiff bay which is nice enough.
Personally i'll be in dempseys drinking, puking and fighting!

I'm sure you haven't.

You're sure i haven't what?

1) Average, its simple really, too bigger ground and not enough people in it.

2) Great in my experience.

3) Its alright... but just alright, a lot of Welsh folk about. Enjoy the local boozers during the daytime though.

4) Pretty much.

5) Yes, theres always a chance.

Fancy seeing a lot of Welsh people in Cardiff.