The Nokia 6 rear camera does well in low-light but it ha its limitations. I've seen many sample images taken with the Nokia 6 in low-light. You can clearly see that the lack of an optical image stabilization poses a limit of what the Nokia 6 can achieve when shooting in dark places. I've read that the slowest shutter speed that the Nokia 6 can reach is 1/11s. This slow shutter speed can obviously help in getting more light to pass through the lens and the camera will use it when needed. The main problem is that there is no OIS, so there is a very good chance that your image will appear blurred out in some degree. You can put the camera on a stable surface and get away with it, but who wants to do that, most of us want to shoot handheld, aren't we?
Night shots shot at around ISO 400 like this image [image removed for some reason] looks really good, but they were still shot with reasonable amount of light. In around ISO 400 you can see that the image noise levels are still pretty good. There is noise of course, but it's not very visible and the image, especially when resized, looks relatively very clean. The main reason for that is that most of the areas of that photo is lit up well, in this image [image was removed] for example, you can see the amount of noise which is apparent in the shadow area (top right side). You can see that the noise is much more visible. It's a great picture because it demonstrates the amount of noise between bright and dark areas in photos. By the way, that images was shot at ISO 376. As you move up the ISO speed, noise levels will obviously increase and substantially.
This image [image removed for some reason] was shot at ISO 681 and if you look closely at the image in 100% scale, you can clearly see that it has a lot of noise and we start seeing a lot of the fine details being diminished and start getting getting really smudgy as you climb up the ISO speed. In low-res it still looks pretty good through. I personally recommend shooting at ISO 400 and below but the Nokia 6 rear camera can still give you pretty good results if you just intend to use the image for sharing for the web. That image by the way still had a good light source that helped light up the subjects in the image. Keep in mind that if you shoot an image where most of the area is dark, the image will look much worse than the one you see here.
The bright F/2.0 aperture lens of the Nokia 6 rear camera help of course, but the 1-micron pixel sensor definitely limits what the rear camera of the Nokia 6 can achieve in dim lighting conditions.
Other things that I've notices is that purple fringing is very severe with the rear Nokia 6 lens (here is an example at the top where the lights are and around the Polo logo above the store). The purple fringing is caused by longitudinal chromatic aberration due to the wide aperture where the red and blue color of the spectrum aren't focused well enough. Of course the Nokia 6 has a prime lens with a fixed aperture lens, so you can't actually narrow the aperture. You can try to avoid having purple fringing by not shooting directly into the source of light, which is what happened in that image that I liked to above. Also note that this can be fixed using software,and it would have been nice if Nokia would have an automatic setting that can auto correct it in-camera so we won't need to deal with it. Most people obviously won't even care about that or notice it, but it's definitely there.
The Nokia 6 rear camera is a very good camera that can take great pictures in good lighting conditions, but it certainly lacking in low-light. There are much better camera out there of course. If you really want a good low-light performance, you should search for a rear camera that either uses dual-lens camera setup with a monochrome secondary sensor like the Honor 8 dual-lens camera that utilizes the added brightness information from the monochrome sensor to improve the image quality. The second option is to search for a camera with a very bright lens, like f/1.8 or f/1.7 and one that employs a sensor with larger pixels, preferably 1.4 micron and larger. Just keep in mind that I ignore the LED flash, because I was talking here about pure sensor+lens low-light performance. Obviously if you shoot a low-light image with the subject up close and the flash LED flash active, you get a better image.
So overall, unimpressive low-light high ISO performance in my opinion based on the Nokia 6 rear-camera sample images that I've seen.
Update: the images that I linked to were removed for some reason. I'll try to find more recent sample images and link to them. If I find, I'll post them here.