Mach was a terrible microkernel because it tried to do too much. Good examples for you to look up are QNX, L4 (esp OKL4), EROS, and Minix 3. These all get stuff done more reliably, securely, and <i>faster</i> than Mach. That they&#x27;ve built so much into the Mach model means anything you do to improve security or performance has to fight with its inherent weaknesses.<p>So, Mach is its own discussion of failure in and of itself. There was also a history... Trusted Mach, Distributed TMach, DTOS... of trying and failing to secure Mach using high-assurance methods. The security improvements in new Mac OS&#x27;s, esp sandboxing and such, were actually recommended with that old research in mind. They realized the foundation wasn&#x27;t going to be secured as it never worked in the past. So, they went for decomposition and isolation schemes for apps themselves plus IIRC integration of TrustedBSD mechanisms.