In many languages, a single word usually may not have both front vowels (vowels that are pronounced with tongue in the front of the mouth) and back vowels (likewise with tongue in the back of the mouth). This rule is known as vowel harmony. To speakers of this type of languages, foreign words that have both front and back vowels are harder to pronounce.
In Finnish, for example, the front vowels are ä, ö and y, and back vowels are a, o and u. In addition to those, there are neutral vowels that may coexist with either front vowels or back vowels: i and e
Classical example of hard-to-pronouce word would be "Olympialaiset" (The Olympic Games); many still pronounce it as "Olumpialaiset".