I have used Dropbox for years to store and manage my entire photo library between two computers. There have been glitches, but overall, it works great and much better than other cloud solutions. I did extensive testing with Selective Sync and determined that it simply does not work with Lightroom. Even with standard previews, 1:1 previews, and smart previews, and preferences set to force LR to use the smart preview for editing, online-only files in dropbox download immediately whenever a thumbnail appears in LR. I was not able to stop it.
My photo library is currently just under 3 tb, so I have a 3tb dropbox plan ($20 per month). My iMac is connected to a large RAID 6 array, and my laptop is connected to a USB-powered 4 tb external drive. This small drive has worked extremely well (knock on wood) with the only downside being that it needs to be connected when I am using Dropbox for other non-photo tasks, which is kind of a pain since it needs to reside on that drive. I suppose I could get a second Dropbox account for that purpose, placing it on my much smaller system drive on my laptop.
Recently, I have moved away from keeping my LR catalog on Dropbox, moving it to an external 1 TB SSD drive which I can simply move between computers as needed. So far, this is working great. It wasn't so much that having the catalog on Dropbox was problematic, and it was fairly fast. But the SSD is much faster and there is no longer a need to ensure that the catalog files are uploaded to DB before switching to the other computer.
I am currently considering replacing my RAID with a different model that is capable of a WebDav connection, allowing me to operate my own cloud-type service, so when I am using my laptop away from my office, I can access my library directly rather than maintaining a local copy as Dropbox requires. So far, however, my tests have shown that speed may be a problem, so I am considering a sort of hybrid solution by using smart previews to edit remotely, and connecting to my server only when I need to do file operations such as uploading, organizing, converting DNG's, etc. The jury is still out on this plan, but it appears to be a good idea.