Hi,
I have many of these motors. I used them at the beginning to learn about stepper driver. It is true that they are heating very much. However, it is not surprising considering that the coil resistance is 30 Ohms per phase for the unipolar witch translate to 19,2 watts at 24 volts. If you use them full step (that is 2 phases always energised) it would be almost 40 watts.
I was using them at 12 volts. They work not so bad at 12 volts, consuming only 4,8 watts per phase. Full step 9,6 watts. Still heating but less.
Alain