Asynchronous generators need to be connected to a line to operate. As soon as you rotate the shaft faster than the synchronous speed you get power out. The power factor is not excellent but it is simple to use. They also operate at variable shaft speed but you get variable power.

If you are off grid, you must use a synchronous generator. On hydro project you can easily control the speed to have the desired frequency. Then you regulate the field excitation to get the right voltage. This is what is being used with engine powered generators. If you use only one generator, you don't need to worry about the power factor. When you operate units in parallel, the power sharing and power factor control becomes difficult. Unless you are extremely competent, you will need to use professional regulators for this.