The thing about security is that it touches everything.
Ease of use? If something's hard to use, it's easy to confuse people into doing insecure things. Security issue.
Unreliable? Security depends on predictable behavior, and failure modes are often unpredictable. Security issue.
Proprietary protocol? Even if it wasn't intentionally backdoored, you don't know its real attack surface, because you don't know all of the verbs it contains. Verbs imply actions, actions imply changes of state, changes of state imply the Dark Side... uh, security concerns. Security issue.
Performance? Even aside from timing attacks and simple DoSing, things often behave oddly when pushed to a limit, especially if that limit involves otherwise-hidden race conditions. Security issue.
Ease of use? If something's hard to use, it's easy to confuse people into doing insecure things. Security issue.
Unreliable? Security depends on predictable behavior, and failure modes are often unpredictable. Security issue.
Proprietary protocol? Even if it wasn't intentionally backdoored, you don't know its real attack surface, because you don't know all of the verbs it contains. Verbs imply actions, actions imply changes of state, changes of state imply the Dark Side... uh, security concerns. Security issue.
Performance? Even aside from timing attacks and simple DoSing, things often behave oddly when pushed to a limit, especially if that limit involves otherwise-hidden race conditions. Security issue.