To be honest I haven’t used it much that way, and I don’t think it’s worth a subscription then as you lose out on the Ulysses-specific organisation and long-form writing features (iA Writer is probably a better choice here then).
I do have it set up for a couple of external folders on iOS & iPadOS, including the Obsidian vault, for when I want to make a quick change somewhere and don’t feel like using the Obsidian mobile app. However iA Writer is also better for that quick edit of a plaintext .md file, at least currently, as Ulysses does not support internal links ([[ … ]]) (apparently support for that is coming).