I don’t see a fundamental problem with composites so long as they are identified as such. People have done that with conventional photography for years. The issue is that in many cases they are not identified as such or are deliberately passed off as “real”. However, it is gratifying to see that with a bit of astronomical experience the brain can often tell immediately that “something is not right” – that’s fine for those of us who have been around for a bit, but not good for beginners, let alone the general public.
The biggest irritation I have is that with some many wonderful sights in the night sky, why bother to fake a picture?
Over-processed images are another matter. But what is acceptable is often in the eye of the beholder, even for “normal” astroimages. Colour selection is another matter – I, for example, am not so keen on the Hubble palette for colour images. To me it feels harsh and unreal – but what is real in terms of images that the eye cannot see in colour anyway?