Seeing pictures of Cardiff McD's afterhours with disheveled, boorish patrons and rubbish strewn about, no-one would want to patronize them: https://hollyhughesgraphics.wordpress.com/2014/02/07/maciej-...
Having used both at normal and at peak pisshead hours, they're both alright.
Not great, not a disaster. Slightly understaffed, and occasionally short on English language skills, but there's not an issue if you want hot food (inc vegetarian and vegan) or drinks at a daft hour.