AviD's Rule of Usability:

Security at the expense of usability comes at the expense of security.

Taken from: Security StackExchange

Very accurate and a good KISS-message in there as well :-)

Not only in interfaces, but also in security, the limitations of us humans have to be taken into account to enable us to live/work more effectively! We need solutions with high usability, as those are the ones that will enable us to put our time to good use. Until we're able to assume complete privacy and security, or the world has been taken over by AI, we'll need good usability.