While I won't say that realism is a solved problem, SD has been able to produce unbelievably realistic photo-level images using "Realism Engine"/NightVisionXL/etc for a while now.
Flux's power isn't necessarily in its ability to produce realistic images, so much as its increased size gives it a FAR superior ability to more closely follow the prompt.
I hadn't seen Realism Engine, looks good. Check Boreal FD https://civitai.com/models/639937?modelVersionId=715729 for the kind of casual photography realism I was talking about. I don't remember seeing that quality with SD.
Also to 'understand' complicated prompts (however far that goes when talking image models). For example, describing two subjects in detail with a relationship between them ("a cat and a man and the cat is standing on the man's head and the cat has red hair and the man is wearing purple glasses") reliably works with Flux, even the 'light' models, but is a crapshoot with SD.
Flux's power isn't necessarily in its ability to produce realistic images, so much as its increased size gives it a FAR superior ability to more closely follow the prompt.