In fact, as action research is intended to produce action, this is usually an emphasis. In that sense, causal understanding actively pursued. Yes, though it is usually qualitative. People communicate naturally in language. Communication is an important part of action research, especially in its more participative varieties.
Quantitative measures can be valuable.
But developing them requires a substantial investment in time. This may not be warranted if you are likely to change your mind about the measures that you need. When suitable quantitative measures are available, there is no reason why they may not be used. Qualitative and quantitative approaches can often complement each other well. For example, you might carry out a quantitative survey or other activity.