Not wishing to stoke this up but yes, there has. When we were in the PL there were a few incidents (against Man United I think) in the ground and there's been scuffles with stewards at the away end.
I walk to matches from Grangetown and during the PL season's big matches Mrs Citizen was targeted with sexual, verbal abuse and no longer wears Cardiff colours. We've seen scuffles outside on Sloper road and there's been more of them. I've seen young Cardiff fans throwing coins at away fans coming out of CCS.
I've seen police drag middle-aged men off each other on Sloper road after matches. It's slowly started to get worse over the last decade imo. There's something wrong when a decade ago we both felt comfortable walking home but feel less so now. That feeling of vulnerability (especially being disabled) is amplified when you see violence or are on the end of verbal abuse.
To put some perspective on this, we'll walk to matches there and back around 30+ times over a season so we get to see and hear quite a bit. I'm not saying it's a massive problem, but I am concerned enough to feel a bit nervous these days when I didn't a few years ago.