UCS2是定长的，固定2个字节，所以不能支持扩展字符，而UTF16是变长的。
UCS2是落伍的。
msdn里有这样一段描述：
UCS-2 is a predecessor of UTF-16. UCS-2 differs from UTF-16 in that UCS-2 is a fixed-length encoding that represents all characters as a 16-bit value (2 bytes), and therefore does not support supplementary characters. UCS-2 is frequently confused with UTF-16, which is used to internally represent text in the Microsoft Windows operating systems (Windows NT, Windows 2000, Windows XP, and Windows CE), but UCS-2 is more limited.