As you like it. I always worry when my data is in a hackable place, but I don't think the hackers of the planet are interested in the little Memento cloud much more than spending their time trying to hack me individually. They can make way more money spending their time on the Google, Apple, Microsoft, and Dropbox, maybe Amazon clouds -- no?
Also, I think Memento uses end-to-end encryption, doesn't it? If so, they can't do anything with it even if they do steal it. Of course, I wouldn't want my only copy of a file to be out there.