Hi,
yes, this is fine as a limitation ( and a very logical one, because a prefab can be spawned in different scenes, so it can not reference anything outside it's own hierarchy).
As you said, simply keep the reference of what you want to reach outside the prefab, or get it when the prefab instantiated itself, using either some logical steps, like Find, or by keeping your ref in a global playmaker variable. Else the game can always fire an event to the nely created prefab with a reference of what it needs to know.
Bye,
Jean