Why coastal-defence ships deter only some opponents

Would heavily armed coastal battleships have been worthwhile for Germany's home defence?

They could deter smaller powers, but not the Royal Navy; building enough to resist a major battle fleet would erase their cost advantage.

Coastal-defence ships raise the force and risk required for an attack, which makes them useful to states such as Norway or Sweden against smaller naval powers. Their deterrent value depends on requiring an attacker to bring a battle fleet. A power that already possesses such a fleet can concentrate enough strength to overwhelm them, so they offer Germany little protection against the Royal Navy. Building a large fleet of coastal battleships would also defeat their economic purpose, which is to avoid the expense of a full battle fleet and release money for other needs.