This may be true, may have workarounds, and may be solved on later Android versions, but it also is not why Nextcloud says they are avoiding the framework. And Google Drive provides the same functionality using SAF, so I'm not sure it's a problem for this use case.
There are no good workarounds unfortunately and it's definitely not solved on later Android versions. I agree though that the reason that Nextcloud gives is factually incorrect (they say that SAF is for providing files rather than accessing them, which is plain wrong).
From what I understand, you can resolve most issues by making ContentResolver queries yourself once you have URI permissions for a folder, including data like filename, mime type and file size in the query projection. This reduces an NxM number of queries to N, where N is the number of nested directories in the tree and M is the number of files in the directory.
Obviously this should be solved with a better API at the SAF layer.
