Things to think about
Let's take out the c word (cloud) and think about what is really happening here. Offsite backups to an external vendor. I can envisage similar risks and potential impact with backup to tape and having tapes picked up in a lorry for secure storage in a remote location.. Does the company storing tapes provide a 100% SLA against data integrity or confidentiality failures? Even if they do an SLA is just a statement that says " if we do bad you get something back" but money can't always replace data. Also, if a 3rd party (for example a government) wants that data what's the difference? (Yes, both tape and online can be encrypted in such a way that you are the only person with the private key). What happens if they have a fire?
My point is that each and every company is responsible for their own data and must carry out their own risk assessment. Allowing a 3rd party to look after you data in a remote facility isn't new, companies have been storing boxes of documents at the bottom of Iron mines for years now and their charging mechanisms have always made restoration more expensive and time consuming than ingestion. What's changing is that people are coming up with different ways of moving that data around
I totally agree with points regarding vendor lock in and the financial stability of the supplier and their hosting centre, that's just means that you need to choose your vendor carefully. I've got a leaky garage at the bottom of my garden that I'd be happy to charge a couple of quid for keeping your tapes in :)
I do work for an online and offline backup vendor and I'm sad enough to have thought about minimising risk and it comes down to money. How safe do you want to be is a direct function of how much you want to spend.
Do full local backups. Keep a copy of the key tapes locally ( for fast restore) and remotely (to protect against the local disaster). Also use an online backup service for real time backup and backup of hard to reach data (contents of laptops of remote and roaming workers).
All the different mechanisms play a part. There is no "best" just most appropriate for a specific task.