Maddy Osman has a great look at content mills. All the same points apply to freelance market places.

It’s the pickiest clients for the lowest prices and you often can’t take the work outside the freelance job platform so you’re only building a portfolio on said platform. If they decide that you’re no longer wanted, you have no business.

She even gives you some good places to look for writing work at the end if that’s what you need.