I'm using a cloud storage service called Hightail (www.hightail.com - formerly YouSendIt). They provide a desktop application, which shows their cloud-folders as if they were local folders (pretty much as One-Drive, Gooogle Drive, and Dropbox), but unlike those 3, the Hightail app is very resistant to Opus 12 integration: it won't link or copy to neither Collections, nor Libraries. The only way I could get it into Opus to be grouped with other cloud-besed services was through Favorites. Will you look into it next time, please? (I managed to achieve my goal via Favourites, but is is still puzzling why it behaves differently to other virtual folders).
I think the main problem is not getting it work with DO, but they are so many cloud-services doing their own thing and always change & break things, that it's nearly impossible or simple too much work to have them all working with DO.
Independent from the fact, that you give away your data to unknown people or tomorrow the service dies and you need another one you can trust.
Collections can only point to real files. Most cloud storage works with real files which the cloud storage software keeps in sync. If this one is different and uses a completely virtual folder, then that will limit what you can do with it. It's a bit like the difference between a directory on your computer, and a web page which makes itself look like a folder, and can be displayed inside a file manager, but isn't a real folder.
Libraries are an OS feature, so if you can't put the cloud folders into a library, I'd expect the same to be true in Windows Explorer, and not be a limitation of Opus itself.