I agree that Prosper is generally improving. From a distance (living in a state that can no longer participate anyway) I've noticed Prosper has made investing somewhat less risky for lenders. A good bit of that is attributed to simply raising the minimum credit score. But also the rating system is improved. A few things were better the old way too.
I think of Prosper not so much as "a viable investment option" as something that can be entertaining and, if you're lucky and/or good, profitable. You have to enjoy searching through the listings for the good ones to do well. Lenders who prefer to simply turn on a portfolio plan and allow automatic bidding without maintaining any attention at all, I don't think Prosper is them and I'd fear for their returns.
I learned a lot about how my fellow Americans view finances from Prosper and was fascinated by what I saw as a lender. I think user LoanChimp coined the term "Financial Voyeurism." Some interesting things to see on this platform, for sure. But I still don't think I'd recommend it to anyone who was looking for "an investment" that didn't come with "a hobby."