There will always be a lot of scam clients and scam contractors on here, but I also think many well-meaning clients and contractors run into problems due to lack of clarity about Upwork policies.
1. Free work - It is against policy for clients to ask, not against policy for freelancers to provide, but 'unpaid test samples' are allowed.
2. IDs, NDAs, W9s - Some legitimate clients will ask for these, maybe thinking they need them. Contractors are under no obligation to provide them, but a client may consider that a red flag.
With these issues, and others, it seems like there is just general confusion and it causes miscommunication between clients and contractors. If Upwork provided absolute clarity about these issues, it could solve a lot of problems. Simply say, "no free work," and then enforce it. Simply say, "contractors on this platform already fill out W9s, do not ask them to provide ID, other documents, etc."
I am suspicious of any client who asks for anything free, even when it's 5 minutes worth of work. However, when clients believe they are allowed to do so, it would make sense for them to keep asking and dismiss a contractor who says no. The same goes for additional documentation. If clients are used to doing this, they will be suspicious of a freelancer who says no. If Upwork were to step in and say, "By using this platform you acknowledge that you don't need an additional ID, W9, etc. from your contractor and we advise all contractors to say 'no' to these requests," then I wouldn't have to try to convince potential clients that I'm doing the right thing by saying no.