Although storing a Case File on iCloud Drive works, and is useful to be able to move a Case File between devices, there can be conflicts which can cause a Case File to get corrupted. For example, if you made changes to a Case File from two different devices when they were not connected to the internet, you would have two Case Files with two different sets of documents, annotations, etc.
Also, Apple offloads files from your iCloud Drive to the cloud whenever they think it appropriate (these files will have a cloud icon with a down arrow next to the file name). There's no way to select individual files to keep locally on the iPad. That means you could arrive in court, or another location without internet access, and find that your Case Files has been uploaded to the cloud.
Lastly, every change you make to a document is backed up to the Case File, which means that if you have a 2GB Case File and simply change the name of a document or add a highlight, the entire 2GB file has to be uploaded and downloaded again.
For these reasons we recommend storing your Case Files in the On My iPad location and changing the Document Storage setting to be On My iPad in the Settings for each app. If you want to move a Case File from one device to another, you can always quickly and easily Air Drop a Case File between iPad devices.