For speed and productivity, it of course always pays to use the right tools.
What constitutes the 'right tools', given learning curves, depends on how often one does the things that require those tools.
There is a common subset of things that Photoshop and Piranesi can both do, but that subset is pretty small and not really related to the point of either product. (And...Piranesi is not a substitute for Photoshop: I agree with Dwight that every AC user must have Photoshop [Elements] and at least modest skills there.)
For photorealistic images from LW in AC or Artlantis or wherever, you can outrageously quicly punch the image up with entourage in Piranesi faster than in any other application IMHO because of two simple facts about Piranesi: (1) it allows any image (or 3D object) to be a cutout that casts shadows and (2) cutouts are placed to scale anywhere in the scene visually and quickly, including ducking them behind the model. You just cannot do that in Photoshop without tedious masking and manual scaling and transformation. In Piranesi, you can quicly populate a scene with trees, shrubs, signage, people, vehicles, etc.
I would never take the time to place such entourage in ArchiCAD itself as billboards or even in Photoshop.
Many things are not worth modeling - for example, ornate plaster mouldings in interiors. How would you display these in an ArchiCAD rendering? You would have to tediously create a material for each wall that had the moulding - unique to that wall height - that includes the photoimage of the moulding, etc. In Piranesi, you just paint the image of the moulding at the desired location, to scale and following perspective - all masking, scaling, etc is automatic.
For non-photorealism, I agree with Dwight, and have said so: many (most?) of the images in the Piranesi gallery are unappealing to me as 'art'. But, this is a reflection of the artistic abilities of the user, not the software. With a pressure sensitive tablet (Wacom), and some artistic ability, Piranesi has a huge set of tools to offer the image specialist.
Just a few more 'pro Piranesi' thoughts.
😉
Karl
AC 28 USA and earlier • macOS Sequoia 15.3.1, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB