Yes, I'd ignored the 2-pin variety. Actually, I am impressed with how the designers of fan motor connections (physically and electrically) managed to keep them all so similar that you CAN connect most to any of the other two and it WILL work enough to ensure your system DOES get good cooling. You just do not get some additional useful features, like speed control and speed measurement. Even the switch from 3- to 4-pin fans has been made easy because mobo fan control systems have been given the options for their output signals to configure to either of those two systems.
The only part of this last item that is still something to watch out for is the "Auto Config" option in this. It appears it works this way: at each start-up the header sends out signals in 4-pin PWM Mode to start at full speed as normal, checks what that speed is, then tells the fan to run at a slower speed. IF that fan does as instructed, it must be PWM fan and that is left alone. IF, however, the fan does not slow down, then it must be a 3-pin fan, and the header switches to the older Voltage Control Mode so it CAN control that fan's speed.
All this works well EXCEPT when you have a PUMP unit of a liquid-cooled system plugged into the CPU_FAN header. Almost all such pumps are designed to run at full speed all the time and thus require the full 12 VDC power supply always. So they are wired just like an older 3-pin fan. That test by the "Auto Config" system thus will detect that this item is a 3-pin fan and set itself to reduce the Voltage to that item (the pump) and slow it down. For this one situation that is exactly what we do NOT want. There is, of course, a simple remedy. One just goes into the CPU_FAN header screens and sets it manually to use the new PWM Mode and NOT try to set itself automatically. In this Mode the header always supplies full 12 VDC power from Pin #2 as the Pump needs, and also sends out on Pin #4 a PWM speed control signal to the pump which has NO connection to that pin and could not use it anyway because it has no suitable chip. So the no matter what the header system thinks the pump speed should be (in its normal operations for fans), the pump runs as designed becasue the header is fixed to use the new PWM Mode.