Wikipedia lists advantages and disadvantages of UTF-8 as compared to a variety of other encodings:
http://en.wikipedia.org/wiki/UTF-8#Advantages_and_disadvantages
The most important disadvantages are IMHO that UTF-8 might use significantly more space especially in Asian languages such as Chinese, Japanese or Hindi and that not all code points have the same size which makes measurements more difficult and many string operations such as search inefficient.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
