Hmm. I wonder why that is. Did you have a hard time with tones, do you think? Or do you think it's just harder to speak idiomatically? I could definitely see the latter.

My feeling — i.e. this is subjective — is that Mandarin pronunciation is far more regular. In Korean, it's not arbitrary, but there are more rules about pronunciation, and pronunciation is more subtle to my Western ears & tongue.

There are rules for consecutive consonants, where some of them disappear or change entirely (e.g. formal/deferential speech level endings, where p/b turns to m). Some sounds also just get dropped or emphasized based on where they are (e.g. words that end with ㅆ, depending on the next consonant). Then there's plain vs tense vs aspirated, diphthongs, etc. I'm still getting a handle on the various particles but there seem to be around a dozen, not including measure words.

Anyway there's one sort of irregular case in Mandarin, which is 3rd tone followed by 3rd tone. Other than that, if you learn how to say the "ie" as in "xiexie" or "bie", neither "x" nor "ie" or "b" really change depending on what they're paired with.

It may be that in practice, a Korean listener can figure out what sounds you're trying to make more easily than in Mandarin. Anecdotally Mandarin seems to have a lot of cognates because many basic words are one or so syllables + tone. Korean seems to me to have more syllables and more particles, which could help disambiguate. But this is just speculation from someone who is by no means an expert on either language.