If they're actually a backup they can be a good idea. If it's a single source, then yeah, it's asking for trouble. I even backup my GitHub stuff locally because who knows what could happen.
Should I go into detail?
Love your profile text, by the way.
EDIT: one quick example is that I’ve lost more data by forgetting passwords than I care to admit. They’ve all been from an age where I thought it was a good idea to encrypt my secret documents, lest prying eyes get to them. Turns out the secrets would’ve been interesting to me, but are no longer accessible. I suspect this isn’t uncommon.
Many cloud storage services try to understand and then "clean up" file types they understand. If you don't believe me, upload and then download a TB or so of images, documents, music, videos, etc. Then, compare the checksums of the original and restored copies.
On top of that, there are services like Amazon Cloud Drive that have content-type restrictions.
The only reasonable litmus test I've seen in this space is "there is a cryptographic proof that some middle manager didn't screw up my backup".
Unfortunately, that means you need to manage the encryption keys somehow. Cloud storage kind of sucks.
This is a you problem. 100 percent PEBKAC.
I've been encrypting every backup, local and cloud, and plenty of other things, for probably 30 years. I've done my share of restores, and have never lost the only key to anything I later wanted back. That's true even though I have forgotten a few passwords, because I had failsafes for that situation.
This requires memorizing only three or four long passwords, which change very rarely, plus a little bit of discipline and occasionally some maintenance of your archives.
On the other hand, putting something somewhere, like some cloud service, without seeing all the implications, is a mistake that's easy to make and hard to clean up. Storing stuff in the cloud in the clear en masse is an insane thing to make into a normal part of your routine.
If it's a true backup, it should be encrypted in my opinion, especially on the cloud. Then it doesn't matter since they can't see it.