Upwork is definitely legit.... I made about $15k there last year.
There's a catch, though. Roughly 90% of what's advertised on there is low-paying crap. If you have a niche and find a few jobs that you can get good ratings on (even if they pay poorly), you'll become more competitive. I can't remember the last time I applied for a job on Upwork - now I only look at opportunities that I'm specifically invited to 'interview' for.
I also have ongoing contracts with a couple of different companies, which means a more steady workflow and that Upwork takes a smaller cut.