I think some people overestimate how good humans are at speech recognition. Unfamiliar accents and noisy environments cause havoc with many people. I had a friend who learned English in India when I was in High School, so I was used to that accent; many of my classmates in College could not understand anything our Indian T/A said freshmen year.
Similarly I had friends for whom English was a second language who had lived in the US for years and were definitely fluent, but had to enable subtitles for a movie in which the characters had a strong southern accent; in general non-rhotic accents were very troublesome for them having only spoken english with midwesterners.
The article mentions the Scottish accent, and I would call that the hardest accent of native English speakers for those in the US to understand.
Similarly I had friends for whom English was a second language who had lived in the US for years and were definitely fluent, but had to enable subtitles for a movie in which the characters had a strong southern accent; in general non-rhotic accents were very troublesome for them having only spoken english with midwesterners.
The article mentions the Scottish accent, and I would call that the hardest accent of native English speakers for those in the US to understand.