But the issue is, that I don't know what exactly are they doing, and don't have means to find out. For example, I don't know whether they provided backdoor, how

But the issue is, that I don't know what exactly are they doing, and don't have means to find out. For example, I don't know whether they provided backdoor, how wide that backdoor is, i.e. at how many places are my Dropbox files, for example.