FUI, they state that for premium users, files never expire due to inactivity.
But I found out that this is not the case. I have 1 year premium and my files are disappearing after about 90 days of inactivity, so it is useless as an alternative secure backup for me.

And due to the way they "count" (or rather not count) the downloads, often you'll get larger download numbers on part1.rar, but lower number (or none) on the other parts. This results higher part numbers disappearing first leaving an incomplete set of files.