Your rule of thumb is mostly correct. It is indeed more common to place the adjective "único" before the noun to mean "only", "just one" or "no more than one" and to place after it to express "uniqueness". Compare:
El mueble está hecho de una única pieza.
Este anillo es una pieza única.
In the first sentence, "única pieza" means "only one piece", and in the second one, "pieza única" means a unique piece, a "one of a kind". Another, more evident example of this difference is:
Me has dado un único regalo. (You've only given me one present)
Me has dado un regalo único. (You've given me a unique present)
"Hijo único" is an exception to that rule, and the only one I can think of off the top of my head. So much so that it actually has its own entry in the dictionary:
- adj. Dicho de un hijo: Que carece de hermanos.
In the right context, "hijo único" can also mean "a unique child", especially when introduced by "un". Compare:
Soy hijo único = I'm an only child.
Soy un hijo único = I'm a unique child (or occasionally I'm an only child).
This sense is less common but possible if the context is enough to disambiguate.