I have been using Upwork for over two years, and am a big fan. At that time I signed up for Upwork and Freelancer.com. Initially I got more offers for work from Freelancer with much less work, but then I saw that over half of these offers were from scammers looking to purchase my identity from me. Yikes!
Upwork's internal review and controls don't eliminate spammers, but greatly reduce it. Instead, I have had to fairly compete on an open marketplace for jobs that are posted and coming in all day every day.
Sure, I had to start with lower prices than the market would seem to be, but I worked hard, got good reviews, and started getting more work - and raising my prices. After becoming a Rising Star and then Top Rated, and maintaining a 100% JSS score (with the help of the ability to challenge the rare poor review) I now find Upwork a healthy supplement to my writing business.