Yes, I believe that the cloud, though it could be targeted, is just as safe as a computer. At least things in the cloud never fail like a traditional hard drive. That's the main point of having the cloud; for those moments when your hard drives go down. I see the cloud as very safe.
Yes, it is safe to back everything up on the cloud, because it is just storing information in another place. The cloud is still keeping information in another place. It is no different than making a copy of a floppy disk and putting it in a drawer. It is just a modern way to do it.
"The Cloud" is just a company who loans a section of drive space on their server, which can be accessed over the web. There is no affirmative law that says that the data you store on their server still belongs to you because it is not tangible property, or "is not property that can physically be touched." This has been it's own debate for a while. Until they resolve this, the company holding your data can use it how they see fit. (e.G. Facebook, Verizon, Google)
The Cloud is relatively new, and we know there are ways to exploit networks and hack data. In theory, time would tell if the Cloud is safe, but in this age of electronic data storage and impending climate disruption, safety is a relative term. It would always be better to have a backup at home, and not have sensitive things stored on the cloud if you are particularly worried about some one accessing them.
I suppose there is nothing inherently wrong with backing everything up on the cloud, with the exception that you could possible be cut off from your data at any given time. I believe cyber warfare is a concern, so I think it is best to keep data backed up elsewhere, but cloud storage can be good for short term uses.