Knowing Node.js alone is not enough if you want to get quality node.js consulting services. Node.js is not capable of covering all features in the development stages. In order for your application to be exceptional, knowledge of several frameworks and programming languages is necessary, check this link for more details. This allows the developer and consultant to reach a higher level, think outside the box, and act globally. The main question here will be, of course, about the stack of skills consultants have. However, you may also ask if the consultancy company provides pieces of training for their employees, what kind of culture they have and if they are interested in providing their staff with a wider knowledge.