The simple idea behind economy of mechanism is that simple is better than complex.
Indeed, the more complex a system is, the more difficult it is generally to secure it.
Another component of economy of mechanism is leveraging existing components: those components are called common controls in contrast with system specific controls.
Using already existing controls generally reduce costs and time to deliver.
Examples of common controls are access control (SSO), password vaults, already available network components etc.
This also applies to staff: no need to hire new people if professionals are already available.
Already available components includes well-known libraries (trusted modules, APIs etc)
Least Common Mechanism
On the other hands, we still want redundancy, resilience and defense in depth. To avoid single point of failure, a balance between economy of mechanism and least common mechanism must be made.
Isolation and compartmentalization is often necessary to reduce blast radius of an attack.
Open design is another exemple of reusable technology, specially in cryptography (Kerchkoff’s principle).
Memory Challenge
Reusing existing infrastructure and system could lead to memory challenges. The reuse of shared/portable memory is the cause for disclosure of sensible information, and corruption of process.
Initialization of data, i.e. starting with a clean field is important to prevent memory corruption. We also want to make sure that after a process is finished, the memory is properly released to prevent memory leakage.