I had very bad experience with Elance some 6-7 years ago. It was really unbelievable, job provider did tried to abuse me (cursing and mind games), I asked Elance to cancel the job and they closed my account because 'thay are losing money on closing $4000 job'. All after making few tens of thousands of successful jobs there. Crazy. I just moved on.

If I understand correctly Upwork is the same team that run Elance? If that's the case it won't work well for developers, stories like this don't surprise me.