unassailableness

Imperviousness is a synonym of unassailableness.

As nouns the difference between imperviousness and unassailableness

is that imperviousness is the state of being impervious while unassailableness is state or quality of not being assailable.

As an adjective assailable

is not defended or not able to be defended.

As a noun unassailableness is

state or quality of not being assailable.

As nouns the difference between quality and unassailableness

is that quality is (uncountable) level of excellence while unassailableness is state or quality of not being assailable.

As an adjective quality

is being of good worth, well made, fit for purpose.