I am also interested in this topic. All my clients from upwork are a start-up or small companies. Jobs are relatively simple in term of mechanical engineering (design). Clients mainly developing some sort of new products and want a 3d representation and technical drawing for production. Those are mainly jobs I got on upwork. Payment is decent, it is great as an additional income, but for a full time... It could work. I am not brave enough to move to full time. Also, there is still a lot of things to learn.