I was never a big fan of sharing cloud data through a unique link, rather than nominating the specific people that can access the data. To me it feels like security through obscurity.
It looks something like this:
All the security of this model relies in the randomness and length of the secret token. But essentially the data is exposed to everyone. Google (Drive) is doing it, Microsoft (OneDrive) is doing it.
Now the really silly part comes in. Because the URL is quite lengthy, a decision was made to use URL shorteners (goo.gl, bit.ly, etc.) to distribute the above mentioned links. Which essentially means that the entropy of secret link is now reduced to just a few characters (around 6 usually).
Martin Georgiev and Vitaly Shmatikov from Cornell Tech did an interesting research on these shortener services to see how much data they can gather, the results were impressive/scary. They were able to trace back Google Maps searches back to individuals and get access to confidential data.