out of 5 flats I only had one quiet one. It was either the neighbour being super loud or in the street outside. I found that the more recent the flat the shittier the noise you hear. They use cardboard as well. I lived in an old walk up and couldn't hear the neighbours at all.
That's why it's better to visit the flat several times, at different times of the day. Earplugs don't work if the entire flat is vibrating when they slam the door every 5 minutes haha