I have been on some freelance website for more than 4 years now and in all of these sites, there is no way either parties can deny payments or service delivered because everything passes through the freelance site. Payment is made through the site even before you start your work and you can only receive it once you deliver it.Sometimes even on freelancer sites some employers will deny payment and give a lot of excuses. Therefore, you have to work with client who has a lot of positive reviews and ratings. You should never work with a new client even on freelancer platforms.