I would agree with this, in that you need to trust the owner to feel "safe". If you have never met the owner or know them personally, then you have more risk. When we rented in October, the owner and I were in constant contact regarding my reservation up until my check in because I was concerned about the cancelling issue.
I have seen on tripadvisor a person's reservation cancelled even though it was booked through Extra Holidays, which is "supposed" to be WBC's own booking. The link for Extra Holidays is right on WBC main page.
tripadvisor review canceled Extra Holidays
There's no way to know if there were other issues with this poster's experience, but it was enough to raise a red flag for me when I was looking at Extra Holidays. Again, just my personal opinion.
Booking.com, hotels.com, vrbo and AirBnB all allow owners to list their properties. You still have the potential to run into an issue, IMO. Koala is another one, I remember seeing/hearing.
I get the impression that the only 100% guaranteed way to stay here without being an owner, or trading, is to book directly through WBC or buy a contract.
I don't own at WBC, I don't have a horse in this race as far as owners renting, but I have seen conflicting info., including on the TUG site where owners communicate. For me, it's not worth the risk.