Operational technology (OT) systems use hardware and software to monitor and control physical processes, devices, and infrastructure - often critical infrastructures. The convergence of information technology (IT) and OT has significantly heightened the cyber threats in OT systems. Although OT systems share many of the hardware and software components in IT systems, these components often operate under different expectations. In this work, several hardware root-of-trust architectures are surveyed and the attacks each one mitigates are compared. Attacks spanning the design, manufacturing, and deployment life cycle of safety-critical operational technology are considered. The survey examines architectures that provide a hardware root-of-trust as a peripheral component in a larger system, SoC architectures with an integrated hardware root-of-trust, and FPGA-based hardware root-of-trust systems. Each architecture is compared based on the attacks mitigated. The comparison demonstrates that protecting operational technology across its complete life cycle requires multiple solutions working in tandem.