It's not that they are a secret. It's that they are sometimes cost prohibitive and lock you into a proprietary, rigid ecosystem regardless of which PLC you chose.
PLCs are awesome for what they are intended for, and they work very well with each other/equipment made with PLCs in mind. but they are an immense pain to "integrate" into an existing non-industrial system. They are basically in their own world, with tooling that's specific to the PLC world, with very little open source support.
I agree that for some one off things like controlling a CNC, they might be more useful than a Frankenstein esp attached to some servo controllee though.
PLCs are awesome for what they are intended for, and they work very well with each other/equipment made with PLCs in mind. but they are an immense pain to "integrate" into an existing non-industrial system. They are basically in their own world, with tooling that's specific to the PLC world, with very little open source support.
I agree that for some one off things like controlling a CNC, they might be more useful than a Frankenstein esp attached to some servo controllee though.