In terms of characters and bytes, a csexp "string" may have any byte sequence whatsoever (because of the length prefix on each atom), while XML (like regular Lisp S-expressions, JSON, and literals in programming languages), requires alternate representations for a few characters (such as "<" and most control characters).
A csexp "string" can include any byte sequence (thanks to the length prefix on each atom), while XML (similar to regular Lisp S-expressions, JSON, and literals in programming languages) needs special representations for certain characters like "<" and many control characters.