I've also heard of good and bad stories about Upwork. I think it all depends on how you handle it yourself.

One of my good friends started his copywriting career on Upwork. First projects he did were extremely cheap, just getting some experience. However, he made sure he does the absolute best he can, with every project. So clients were happy and recommended him to other clients. My friend would be doubling his hourly rate pretty frequenly, and eventually stopped using Upwork altogether, because clients would contact him directly, by reference from his previous clients.

On the other hand, I know a person who's been copywriting on Upwork for seven years, and still has an hourly rate of 10 USD / hour, which is just ridiculous.