TL;DR UpWork has it’s problems, but you can find great clients and freelancers on it.
I’ve been on UpWork (formerly oDesk) since 2009. It’s been great for me! I also signed up on Elance in 2010, but I was never able to land a job there, so I stuck with oDesk/UpWork.
There are many people, freelancers and clients, who have issues and bad experiences on UpWork. However, my view is that it’s an international marketplace, and it will therefore magnify the best and worst of freelancing - you’ll have good clients and bad ones, great freelancers and poor ones. A failed project, or a bad experience isn’t necessarily an indictment of the platform.
That being said, the merger of Elance and oDesk into UpWork has been fraught with technical problems. Personally, I’ve had no issues whatsoever on this platform but I will not discount the fact that other have had issues. However, the people who manage the platform have accepted that there are problems and are working on them.
Overall, I think it’s a great platform and you just have to make it work for you.