The difference with photographic paper and film is it is expected to produce very predictable and consistent greyscales and colour balance - decades old film will still work but will look different to brand new stock.
PCB coatings are essentially a binary process, and any variation in processing time due to age etc. isn't a problem as the develop process is closed-loop, i.e. you pull it out when it's done.
The worst I've ever seen is on one particular piece of cheap material that was maybe 20yrs old, where the cover film adhesive had dried out so it took patches of the resist off with it.