The problem is that the new system can vary in ways that the control system wasn’t designed to handle: it can get into states that aren’t modeled by the original system. This is what the Law of Requisite Variety is about: if you want to build a control system, the control system needs to be able to model every possible state that the system being controlled can get into: the state space of the control system has to be at least as large as the state space of the physical system. The fundamental problem with building software control systems is that we humans aren’t capable of imagining all possible states that the systems being controlled can get into.