Composites cause much heated and passionate debate, particularly across the social media platforms!!
Personally, I feel that some composites are "ok" and some represent digital art creation, rather than photography, but this is just my opinion. My opinion is no more or less valid than anyone else's.
For nightscape photography, the composite is likely to involve combining a night sky image with a foreground; each taken at different times, possibly at different locations (maybe even different countries), and possibly using different focal lengths. In this way, composites are totally different to the exposure blending techniques described above. A composite could, therefore, comprise a milky way image captured over the Atlantic from south western Portugal, with a mountain landscape captured in the Lake District in north west England, UK. Is this ok? Is it the image that matters, and not how it was captured/created? Does the image have to reflect the reality of the scene before the photographer at the capture stage? It is all a matter of personal feeling and opinion.
Personally, on the rare occasion I create a composite nightscape image, it will be a deliberate decision taken for various reasons. It may be that I have set out with the intention of capturing an image of the milky way above a particular landmark, building or object, but when in the field, the clouds have conspired to thwart my plans and the heavens are simply hidden from sight. I can still, however, capture the foreground with any light painting that I had planned. Back in the editing room, I can replace my cloudy sky with a shot of the milky way taken on another night. With the latest Ai sky replacement facility provided by the likes of Photoshop, this is very simple to do.
If I were to decide to go down this route, I would make sure that the milky way shot was taken as close to the date and time of the foreground shot as possible, and from the same angle. Other photographers will not worry so much about this, unless the landmarks are very well known, and it would be obvious to an expert that a particular shot of the milky way would not be possible from the angle depicted in the foreground.
It's up to you.....but be careful would be my advice. I have a small gallery of composite night images that I don't particularly like, but have proved to be very popular.