During my time on freelancer.com, I only participated in contests.
These contests were absolutely horrifying. Nearly every user who entered had (1) completely ignored the guidelines set by the contest host, or (2) essentially copied the top-rated designs and then spammed the entry. Then they create fake accounts to "like" their own images & make them more visible.
But by far, the worst offense was that a ridiculous amount of users just stole images and logos from other sites, and did so without consequence.
I actually had a contest where I was the first to enter, I corresponded with the host to improve my design, and I spent upwards of 8 hours making something they would like. But when my design became the "top pick", a good 10 or 15 designers went and copied the image almost exactly. Even so, I thought I was fine because I was still rated the top design by the time the contest closed.
And then another designer, who was clearly using images stolen from google & photoshopped (mind you, the guidelines stated it was to be an "illustration"), was awarded the prize at the last minute.
This was not Freelancer.com's fault, it was the host's decision. Regardless, that was the last straw for me. I'm not about to spend hours on an original design just to have the winner be someone who slapped together an image in 5 minutes, especially if they just essentially copied my entry. And I'm certainly not going to continue making designs for employers who couldn't care less about anything other than the end result, even if it's created illegally.
If thousands of users on a website consistently violate copyright law without consequence, why would I feel like my work was safe from suffering the same fate?
From what I've been reading, I'm at least thankful that I never gave them access to my bank account.