I was pulling around $2k-$3k/month on Upwork as a top-rated writer. I ended up getting banned when a couple of clients I'd taken off-platform messaged me when I disappeared for three weeks in May.
I actually make the exact same, plus 5%, now because I kept all of my clients. Go figure, when you work with someone for a couple of years you end up exchanging contact information at some point.
Like anything, it is what you make it.
I liked being able to pick up new clients easily and the rates were generally better than working with Freelancer. For whatever reason I never seem to able to qualify for content mills in general(after three years in the field and a host of happy clients behind me) so now I'm working with old clients and applying for agencies.
If you're working as a writer right now, however, you're not going to be able to join Upwork. They've been routinely denying applications for writers and SEO experts for about a year now.