Designer is the most useful for editing PDFs out of Dorico. You can manipulate individual elements, e.g. staff lines, noteheads, etc.
It’s also good for making SVGs if you want to create graphics for importing into Dorico.
Publisher is capable DTP package, and you can incorporate PDFs from Dorico within larger documents very easily. See my post here:
Photo is very good for photo and bitmap editing.
The three apps together have a ‘synergy’ (dread word), so that you can do photo editing of an image in Publisher as if you were in Affinity Photo, without leaving the Publisher app.
I’d say they have about 75% of the functionality of the Adobe equivalents (and the missing 25% is towards the rarer end of the feature set); and you can buy the whole lot “for keeps” at a fraction of the cost of renting Creative Cash Cow.