Mega offers true end-to-end encryption, a responsive smartphone app, and great upload speeds (tested in Europe). However, the service has an essential flaw: The desktop app allows accessing the user account on the Web without additional password verification. That means that if an attacker can get hold of your unlocked computer, he can access your Mega account. From there, he can delete files irreversibly. He can even download the recovery key. With the recovery key, he can switch off 2-factor-authentication, and take complete control of the account. In other words: When your computer is compromised, so is your data.
That may seem natural, but then again: what is the purpose of a cloud backup if it is no more secure than your computer? What Mega should do is require the password (or at least the second factor of 2-factor-authentication) before granting access to the user account. I explained them the problem and the proposed solution, but they replied saying they do not believe Mega has a problem.
I have not tested for the desktop app backdoor. However, iDrive fails on another criterion: it does not allow collaborating with other users. First, it does not allow sharing folders with other users in order to work together on the same data. Second, it does not even allow sharing download links to files and folders (this works only for non-end-to-end-encrypted objects).
You may want to ask why it makes any sense to share data that is so private that you keep it end-to-end encrypted. The answer is that there is data that you want to share with a friend, but with no-one else. That is where end-to-end encrypted sharing comes in handy.
Furthermore, iDrive keeps only 30 versions of your files. That may sound like a lot, but if you work on any document for an hour, and you have the habit of saving it frequently (as you should), then chances are that you cannot go back to how the document was an hour ago.
Sync offers all the security features that iDrive and Mega offer, but it does not suffer from their drawbacks. You can share files and folders, via link or via collaboration, while keeping the end-to-end encryption. And the desktop app does not let you through directly to the user account (the second factor of two-factor authentication is required).
The drawback? The mobile phone app is painstakingly slow! (At least when you access it from Europe.) Navigating into a folder can take 10 seconds — meaning that going into some 6-level structure takes easily one minute. Also, the interaction with the native Files app on iOS is buggy and fails more often than it works.
I personally stay with Sync.com. While the app is slow, the service supports at least all the security features that I need. Your own preferences may differ, of course.