Ben has a good guide on using an iPad Pro as your only computer. I've been iPad first for a while now and have General;y been very happy.
He talks about not using Dropbox or Google Drive with an iPad Pro and I can concur, mostly. The problem with iCloud is that if you have a Mac around as a server like I do then you may be limited on how much you can actually store in there.
Unlike Dropbox iCloud doesn't allow any selective sync on your Mac. So I have a 512GB which means I can only store 512GB of files on iCloud. That may seem like a lot, but I'm doing more and more video stuff so it's really not that much. iCloud needs offline only folders on macOS.
The second reason that Dropbox seems to be a poor choice is that it seems to eat your storage. I have had an 800gb System storage report in iOS, and I'm not the only one to have this problem. Where many others found that they had to wipe their iPad a recent update to iOS showed me that Dropbox had 1.4TB of storage on my device.
No there is no iOS device that has that much storage but it at least let me know that Dropbox was likely the problem to an overflowing system storage number. Sure enough removing Dropbox from my iPad took me from 900GB used to 100GB used.
If you have a huge system storage report in iOS and use Dropbox, remove it and see what happens.