Software companies regularly rely on third-party platform providers to host code, build servers, web servers, and apps. These can be a source of vulnerability. The Linode web host was compromised in 2012, leading to the theft of bitcoin from several services that were clients of them.
Google’s Play Store and Apple’s App Store are potential points of failure. They could remove the Casa App from the platform at any time, or potentially be compromised and enable the app to be replaced with malware.
Code base is auditable, requires cryptographic signing of every commit.
Two-man rule required for code commits.
A system with a mix of different hardware and software provides checks against pieces of the system becoming untrustworthy.