I have several Macs for different points at my work and most of the work files are on iCloud so sync across the Macs. I have a 2T iCloud plan. I have dropbox also partially due to historical reasons and partially to use it for files I sync with others.
Dropbox syncing is rock solid and fast. It has never failed me. iCloud app synching (with their app specific folder) seems to work ok, creating ones own folder and files on iCloud is a whole different matter. Many apps are not iCloud aware and so if you have the storage optimization turned on and a file your app needs is not actually on your Mac, your app may have an issue. Not an iCloud problem by itself but if this occurs it is not always clear where the problem stems from and before you know it you are fixing something that requires just a download. But the “just download” (i.e. click on the little cloud icon next to file in finder) often times does not work or works very slowly. I have had to wait for some files to come across for many hours. This is such a problem to me that I turned off storage optimization all together.
A few weeks ago my SSD on one of the Macs went poof. Luckily I had a same size SSD and a backup of a week earlier. Funny enough, I have been backing up for many years and this was my first SSD failure (ha!). I had to fudze getting the OS back on the SSD, the backup took a few hours and I was back in business. Not! The backup included my ~/iCloud folder (which maps to the mobile sync folder) and the OS started a reconciliation process between what was just restored and actual on iCloud. Still not a bad thing, however, for some reason macOS started to replace newer files on iCloud with older files from my restore job. That turned into a big mess. Notice that I do have dropbox on the restored machine and dropbox too started a reconciliation process which took all but a minute and deleted nothing from the pool but did copy some missing files from the pool.Exactly as one would expect.
iCloud for phone etc. backup is the bomb, for app specific use seems ok, for general use I have a hard time recommending it.