Had similar issues with oDesk, which is now as I know is owned by the same company as freelancer.com. Though it has time tracking and I always tried to bid only on projects with hourly rates, I got stuck with non paying client which dissapeared from the face of the earth, and even when submitting that project is canceled from my side because of no show they are not closing the project. Not to mention I was not paid at all. After this incident I decided to move away from oDesk and similar platforms. As the hourly rates are low, and getting repeating clients with whom you already have relationship to work outside oDesk is easy.

Also there is an issue with 3rd world developers who are making impression that freelancers should be paid dime on a buck.

From client side there are different horror stories when clients are scammed into accepting low-rate bidder, and get burned heavily, and then either turn back from these platforms, or turn to accepting only bids from local country.