Style transfer commonly hallucinates objects or adds artifacts. Making something look like a painting is actually much easier than making it look realistic. In paintings, artifacts are often tolerated as 'artistic'. In a photo, it just pops out. Also, style transfer approaches are not temporally stable most of the time. Have a look at the comparisons with state-of-the-art image-to-image translation (CUT, TSIT) and photo style transfer (WCT2). In the case of GTA/Cityscapes shown here, most of these methods put trees in the sky and/or flicker. Also the use of G-buffers allows a much deeper and more robust translation than just using images as style-transfer approaches.