I have never attempted a full restore with TM. Dr Google tells me DB will backup an external drive, but it beats me how this is done for an existing drive (plugging in a new one apparently brings up the dialog I want). Oh well. It isn’t worth the effort to get it sorted. I prefer to have physical backups in multiple locations anyhow.
For me, if I have trouble, it’s usually with a file or two, and so far TM has been hugely helpful in that regard. I’ll give a think to CCC.
That is an interesting feature of Dropbox - I am not certain if it is truly a “backup” or a “sync.”
Plus seems to be it’s cheap if you already have Dropbox and it doesn’t need much setup.
Downside seems to be limited control over the details of how often to back up as well as retaining past backups.
Well, I haven’t figured out how to get this supposed feature turned on yet. It is only Todo list for “someday” when I have time to hunt the snark. It may end up being an elusive boojum I’m in the end.
Everything seems to be working fine at the moment, so this doesn’t appear to be an immediate concern. Still, just in case, I do occasionally copy the entire database onto another external drive a few times a month, just in case (admittedly a rather primitive method that will not catch anything made or modified between backups). It makes sure a few hundred gigabytes of important data doesn’t entirely disappear if something catastrophic occurs while also reminding me each time that I may want to look into that CCC thing 
Twice in 10 years for a full restore, and it worked
If it failed, I would fall back to alternate backups
I test the backup occasionally with individual files but
The condition of any backup is unknown until a restore is attempted
(Schrödinger’s backup)
True - and of course the condition of any file is unknown until access is attempted.
But there is a notable difference I have observed with Time Machine vs. CC. If I clone a data source with CC, then the files exist in the destination in the same format as the source. Thus barring some catastrophic hardware failure in the drive or controller, I can recover or view any file independently.
While I do not fully understand the data structure or digital housekeeping involved, it appears that Time Machine uses a methodology for its incremental backups where a file (or most of the backup) needs to be reconstructed before viewed. Perhaps it makes things “efficient” during backup by placing file edits in a different location than the original file so it then needs to re-create your edits before it serves the file to you?
Maybe others here understand the file format and data algorithms for Time Machine better than I do and can help out. But it seems to me that with Time Machine, there are additional “housekeeping” files or housekeeping tasks which make a Time Machine Backup more fragile than a data clone. If that housekeeping data fails, then access to the entire backup fails.
I’m not a CC user
For a “clone in the same format as the source”, I do a weekly export using DT’s File>Export>Archive.
A zip file saved in the cloud, with a few versions on an external drive
Is DT the only data of significance that you back up? And it would not be a major issue if you lost the most recent week’s data between your weekly exports?
Yes, the DT databases hold all my “significant” data, however Time Machine backups up my entire drive
And it would not be a major issue if you lost the most recent week’s data between your weekly exports
Yes, a serious issue
My iCloud sync store would bring my data up to date automatically
But sync is not a primary means of backup. The same problem that causes your main SSD/HDD to fall can affect your sync too.
I think the utility of the system you describe depends on the nature of the information you are storing. If you are writing a novel and worst-case you can just redo your last week’s writing or if you are storing personal tax records and worst-case can reconstruct the last week’s transactions from your bank statements then no problem. On the other hand if you run a business and are storing critical documents for client work, then it sounds as if you could lose important stuff.