Principle (Observability): Pull is more observable than push.
The primary control in a pull system is the WIP level (or WIP levels, in a system like Kanban that controls WIP at each station). In contrast, the primary control in a push system is the release rate (e.g., as set by the Master Production Schedule). Hence, in pull we set WIP and observe throughput, while in push we set throughput and observe WIP.
In pull, the control is easy to observe, since we can physically count WIP. However, in push, the release rate must be set relative to capacity (i.e., it is really the utilization that we are controlling). Capacity is not easly observable, since it depends not only on the base rates of the machines, but also on detractors like maintenance, failures, material outages, operator unavailability, etc. In general it is very difficult to make a precise estimate of capacity.