As a native Mandarin speaker, I always think the most difficult feature in English (and a few other European languages, like French) are the rich vowels. Like done vs down, beat vs bit, trailing dark l vs -ou/-u sound, and frequent vowel reduction in speech. Even worse, different English dialects randomly shift vowels (maybe like how Mandarin dialects use different tones). Neither my ear nor my mouth is tuned. From Wikipedia "English phonology":

> The number of vowels is subject to greater variation; in the system presented on this page there are 20–25 vowel phonemes in Received Pronunciation, 14–16 in General American and 19–21 in Australian English.

Native English speakers, if they are not teachers, tend to underestimate the challenge. I see YouTube videos that the western Chinese learner hypothesizes Spanish is most difficult for Chinese to learn because of the RR consonant -- I learned Spanish casually for a few years and I disagree. RR is difficult to pronounce, but I can clearly hear it and I won't confuse it with a different sound. In contrast to English, Spanish vowels are so easy.