I'd like to use an EV3 brick to power an old 9V motor (74569). To do so I'm going to get one of these conversion cables (x1676) but I'm not 100% sure if it's possible to control the speed of the motor with it. I'm aware of the limitations of these cables but RPM control is crucial to my project. These cables became quite expensive nowadays so I don't want to waste money on them.
You'll be able to control the 9V motor as an unregulated motor with the similar named motor block, you'll be able to set the speed approximately, but any accurate RPM control is out of the question as there is no feedback pulse signal from the motor to the EV3 telling it how fast it is going.
Also there are some programming quirks with this cable and an EV3, like having to specify two motor blocks as the first one is ignored, but I don't recall the specifics.