My experience, fwiw:
Upwork--will verify your citizenship and tax papers in real life before you're earning money; this is not necessarily bad but so far I've not scraped up the money to send them the papers!
Hirewriters--good site for competent writers UNTIL you earn enough money that they have to file proper tax papers, which they refuse to do.
Iwriters--nice site in some ways, but they use "i-frame" coding, meaning that in some parts of the world (like mine) clients won't receive what you wrote. Local servers scramble anything coded between "i-frames," so if I wrote 1000 coherent words, the client would receive 600 coherent words followed by 100 gibberish words, and would naturally complain. So I check in now and then but haven't written anything there for two years.
Fiverr--went crazy after some efforts to "update." They had no problems with me or my work, and in fact have e-mailed asking where I've been, but they now have an "on site or on vacation" system that automatically accepts jobs, or tells people you're "on vacation," while you're offline for the night. I'm not trying to work with that. I hope they'll fix it.
SEOClerks--nice forums, but more for botmasters than real writers or coders.
WordClerks--nice forums for writers, but hardly anything goes on there.
Patreon--where some people were doing well, until they decided to "upgrade" to a Windows-10-based system that locks out computers I choose to use, so to Flint with them!
Mturk--apparently never liked my computer. I e-mailed for an application form and got a reply that I wasn't qualified.
Guru is where the action seems to be. I've had no problems delivering work or getting paid, but clients have had problems trying to pay for the kind of cheap little gig people do on Fiverr. Guru will process payments below $25, but they're surly about it and won't allow writers to offer them except in batches that add up to $25.