I really have the feeling that DropBox will die. They don't do anything Google Drive, Microsoft OneDrive or Apple iCloud doesn't do, and these companies all have much more money / talent / infrastructure / synergy with other products.
My concerns have less to do with individual files being manually added to a Google Drive or iCloud folder and more to do with how well it handles starting entire new project folders w/ hundreds (or thousands) of files that are constantly being updated.
Used to be that way for me too until all the nasty tricks they employed in the OS X app installation process to elevate their privileges came to light. Though the syncing still works as reliably I no longer trust them with the content or to behave responsibly.
I haven't had any issue syncing with OneDrive or Google Drive for larger projects. I don't doubt you might've run into that issue in the past but I'd recommend giving it a go if that's what's holding you back. If anything knowing that you can reliably switch storage providers in the future should you need to is a good card to hold.
Google and to a slightly lesser degree Microsoft have horrible histories of simply turning off successful services. Or "improving" them by replacing with something objectively worse.
AFAIK, neither Google Drive, OneDrive, or iCloud will work on Linux.
Third party though? onedrive: https://skilion.github.io/onedrive/
google drive (assuming you're using gnome): https://www.howtogeek.com/196635/an-official-google-drive-fo...