I have not had any problems to copy files from and to iCloud Drive. My problems consists only in how they are organized.
I work with legal cases. I want one folder per case - not on folder per filetype.
I also would like to mix different type of files on my folders. Invoice is a FileMaker file, correspondence is .Pages and .pdf, investigations .numbers, and then some normal pics and photos.
I would LOVE to have all my info inside the Apple eco-system but I can not today. Using Devonthink PRO office and are sorting all my files in to a large database, folder by folder for me case by case on my MacBook PRO and then I just synch the entire database with my iOS devices. That way I have all my documents sorted as I want. The negative is that I can not make changes to the database copies on my iOS devices, just read.
But again, I save a lot of cellular data, syncing takes 3 seconds via WiFi and most important - the files stay in the folder I want them regardless of filetype.