I used to love Dropbox's Smart Sync feature, but it's gotten very frustrating since it started automatically removing files that it thinks I haven't accessed in a while. I find that it regularly purges files that I use ALL the time (like our company's main art library folder that contains frequently used digital assets). It may be doing this because I usually access the files indirectly... For example, an After Effects project may have hundreds of "links" to those art library files — so if Dropbox decides to automatically purge those files, I can't work on the After Effects project until I go through the tedious work of redownloading everything — and then repairing whatever broken links remain.
If I've selected files to "make available offline," it would be great to have the option to KEEP those files on my hard drive indefinitely. I realize this was probably intended to be a feature (not a bug)... But Smart Sync hasn't always done this. I used to be able to choose "make online only" or "make available offline" without Dropbox making those decisions for me at some unknown time interval. Hopefully my example above helps illustrate why this is very bad for our productivity.