In their article Is Dropbox secure? 3 weaknesses you should know about, they claim that one of the weaknesses of Dropbox is that Dropbox owns the encryption key on their servers. So if Dropbox gets hacked, a hacker can read your files.
However, I tried Sookasa and I was never asked for or given a key of my own. So doesn't that mean Sookasa do exactly the same thing? (i.e. keep the encryption key on their server.)
So the only thing that changed is that a hacker now needs to attack Sookasa instead of Dropbox to read your files. And instead of trusting Dropbox with your encryption keys, you now trust Sookasa with your encryption keys.
Thus, does Sookasa really make Dropbox more secure this way?