Not Upwork. Definitely not Upwork. They used to take just 10% off the top. Now it's 20%, and gone are the days when you could bid on an unlimited number of jobs. You get a handful of bids a month and if you want more you have to pay for them.
Worst of all, experience means nothing on their platform. They make a big deal about the work hours you accumulate but it doesn't really make you more successful. You can have thousands of hours on your profile and you'll still be competing with freelancers with barely a few dozen hours.