SmartSync is a great idea. Saves a lot of Disk Space. Yeah.. But it does not work reliably. Not for me at least. Your mileage may vary.
I have 3 Dropbox Plus accounts and about a dozen free accounts. I do share folders between those and finally I sync onto my own Dropbox Plus account. Not so much on data, but in my dropbox account it's about 120GB. I have some additional data over on OneDrive (Business). About 200GB. There are historical reasons for that.
Dropbox client version 85.3.138 on all involved machines. I do have 2 Windows 10 at work, one of it virtualized with limited disk space. In addition a business laptop and yet another virtualized Windows 10 at home.
Here are my experiences with SmartSync:
On a machine where I do sync everything (120GB), I selected some larger folders to be online only. Worth about 60GB, I should save 50% on disk space. First of all, it took about 2 days until Dropbox would stay quite. In this 2 days, I won about 30% on disk space.
In OneDrive, I do have all of the data only online. Just to compare, I selected about 50GB to be offline on my disk. That took about two hours to download. Then I selected those 50GB again to be online only. This took about 20 Minutes.
Looking at the remaining Dropbox files, they still take quite some space. Between 1% and 20% of the original size. Probably depends on the file type.
The OneDrive foldersize reports 100% local, even if the files are just online. TreeSize Free showed me that the actual local disk space was nearly zero.
When syncing occurs and SmartSynced folders are involved, explorer starts to flicker until the sync is done. During that time, I usually have to open another explorer window to continue to work.
Switching a folder from offline to online and vice-versa needs a lot of patience. The effect is not immediate and might take up considerable time, even to start.
With OneDrive I see immediately something happening. I see the syncing, thereas in Dropbox I just see number of files counted up or down. After some time..
As I have limited disk space on the virtual Windows 10 machines, I do only selectively sync some folders. As per instruction, I selected additional folders that should become visible, but stay online. This seemed to work and I went away. Some time later, I got an alert that diskspace on the virtual machine was full. When I checked, I found that Dropbox had not kept the additional folder online, but actually downloaded it. Twice! There was an additional copy of each file in the hidden .dropbox.cache folder. I was not able to recover the space with SmartSync settings on the folder (set to online only). This either took too long or just would do nothing. I was also unsuccessful with de-selecting the folders in the selective sync config screen. It would just not give up the local disk space. At the end, I had to disconnect the account from the local dropbox, then manually delete the files, and finally reconnect again and let the selected folders resync again.
Even today, on my main PC with plenty of disk space, SmartSync seems to switch itself on or of on certain folders. Very annoying.
Today I lost hours on something I have to admit, is partially my own doing. Or my stupidity perhaps.
Instead of using Sandboxie and Virustotal, I executed an unknown file on my disk. Fortunately it did not contain a virus, but it produced a strange error. Concerned, I wanted to start some troubleshooting tools (which are located in one of the dropbox folders).
I was not able to start them. The system could not access the files. (dropbox was happily running, by the way). I downloaded fresh copies of the tools from the (sysinternals.com) website, and then I checked several other files, then folders, then found that none of the .docx, .xlsx, .txt files were working anymore. Not all of my folders were acting like this, thus I made a wrong conclusion. I assumed I had stopped an ongoing cryptovirus when I killed some flaky processes.
After hours of troubleshooting, restoring backup, always finding again the files not accessible, it dawned on me... The "crypted/damaged/whatever" files were always only in the dropbox folders.
SmartSync has shown some hidden talent to confuse me. Actually, what I found out is that SmartSync/dropbox made those files not accessible for me. No file was encrypted. Just dropbox did not let me access those files and folders. Shame on me.
I disconnected dropbox, removed all the local files, connected dropbox again without enabling SmartSync. Now all files are accessible again.
I won't use SmartSync anymore. It just does not work as expected.