re: "What advantages do Upwork have over competitors in your opinion?"
If you are a client, the biggest advantage is that the freelancers here are better than what you will find on competitors' sites.
If you are a freelancer, the biggest advantage is that Upwork offers the best opportunity to earn money. There are more real clients on Upwork, and Upwork's system is based on making money through freelancers earning money. The presence of paid connects makes Upwork's system less "pure" than it previously was. But it is still a relatively pure, true system rather than something of a "scheme" which some competitors tend to be.
Besides what Preston mentioned, and i fully agree, I would also add
The design of the Upwork interface in itself. It’s very well balanced between functionality and aesthetics.
And also the community which is extraordinary helpful.