As mentioned, the video and audio limits StreamFab encounters are the same limits a web browser user has. So for example, Disney+ limited via web is limited to 720p, so StreamFab can only download from Disney+ at 720p. (note: there were some leaked keys for older DRM used on older content on Disney+, so there is some 1080p, but new/current content is done with a different DRM version and limited to 720p), Neflix can download 1080p for most content, but if its from Sony or one of their studios like Columbia, its limited to 480p/540p. And such..
Due to the DRM level used by Hulu and Max, you can download some of their content at up to 4K.
With Hulu its mainly "Hulu original content" (stuff like Only Murderers in the Building, etc), and you wont find movies and series from broadcast TV channels as 4K.
With Max its select content, and its relatively limited. Its also usually content added to Max before a particular date, as newer content appears to be encoded with a newer DRM.
Apple TV+ is another one where there is some older content available for download at 1080p, and very few that can be downloaded as 4K, and more recent content is limited to 360p/480p