Perhaps the biggest potential issue I see to cloud backups is the problem of a mis-configured storage bucket (or whatever term the provider uses.) We've all read the various stories of companies that have put data of some sort into a cloud storage area that was mis-configured and had no security set, allowing anyone on the internet to grab themselves a copy of the data.
Obviously, backing up / storing your backups in the cloud have some fairly massive advantages. You can get disaster-resistant storage (in California? Store your backups in a data center in the Midwest or East coast so when the big one hits, you still have a way to recover,) you can get at your data a lot faster than if it's in a warehouse somewhere, etc. But, and it's a big but, you also better make sure that cloud storage is secure so you don't find your company on the news as having had a data leak with you playing the role of the sacrificial goat.
My preference, if I were backing up to a cloud storage, would be to automatically assume that the cloud storage *IS* setup insecurely and then to take measures to protect the backups as much as *I* can from my end. I would take every possible precaution I could so that if / when my backups leaked out to the world, they'd still be non-restorable, at least without using whatever method I used to protect them initially to "in-protect" them. Arguably, you'd even want to do this with a 3rd party application / device, because who's to say *they* don't make a mistake?
I do see the advantages of cloud backups, but you do need to be aware of the potential pitfalls as well, just as with anything else...