Using a Unicode character is more reliable than using a formatted character because it stays the same everywhere. Formatting can disappear when you share, print, or open a file on a different device or program. A Unicode character, however, looks the same everywhere.
Therefore, the Unicode raised ᵘ will always show up correctly, while the superscript “u” might not.
For more information on how to find and type the Unicode raised ᵘ: