When the mechanical drill is connected to rotation to mine a 'motherload', the increased stress requirement does not take effect if the drill is newly placed. That is, if the mechanical drill block has never been connected to rotation before, the increased rotation requirements to drill a 'motherload' will not be checked when it is first connected to rotation.

For example, the below setup only produces 320su of stress capacity, but the drill is still able to run. Note that the tooltip shows the correct stress requirement for the drill, but the actual available stress of the network is too low for it to run, despite it doing so.