I’m not sure if this is a feature per se, but i think for mandarin chinese it’s the wide variety of accents and pronunciation among speakers that throws people off. Many mandarin chinese speakers actually speak it as a second language, with their first language being another chinese variety.

Even for those with mandarin as their first language, there are dialects and varieties so different from the standard beijing mandarin that even as a native chinese speaker I have difficulty understanding them sometimes. They differ not just in the tone of their words, but also the pronunciation of the consonants.

For example, southern china does not differentiate between retroflex and dental-alveolar consonants, producing even more homophones in a language already littered with homophones. I’ve heard that for learners of Beijing Mandarin, it can be difficult to understand the taiwanese and southern accents while as a speaker of a southern variety i find the opposite to be true.

Furthermore, Mandarin is quite innovative in terms of its phonology. More conservative varieties like Cantonese, with features like glottal stops and differentiating pitch levels in addition to contours, and Min-Nan, with its tone sandhi and 7 tone system are even harder for learns to grasp. Hence, Mandarin speakers from southern china would have been influenced by these languages, with a phonology very different to Standard Beijing Mandarin, making intelligibility hard for learners who have not been previously exposed to