The Nokia 6 lacks an optical image stabilization for its main camera. I think many Nokia fans expected this first Nokia Android smartphone from HMD to come with an OIS like the Nokia 1520 and the 1020 that had one. An OIS could definitely help produce better low-light images, especially considering its very small 1-micron size pixels that really just prevents the Nokia 6 to really perform well in low-light. The fast f/2.0 aperture lens of the Nokia 6 camera does help, but with such small pixels (due to a small sensor and a very high-resolution), we can't expect this phone to do well at high ISO speeds.
An OIS could really help here. Why you ask? because the main benefits of an optical image stabilization is that it allows the camera to capture images at slower shutter speeds, slower than the recommended for a sharp image, and still get a sharp image with good exposure. This also allow the photographer, when shooting in manual mode, to prefer a slow shutter speed over a high ISO setting, resulting in images with less visible image noise. A very good OIS can allow up to 4-stop compensation. This is super important feature for mobile phone cameras that compared to more advanced digital cameras, have rather small sensors.
It's not that it's impossible for sensor manufacturers like Sony or Samsung to create a sensor with 5MP or 2MP resolutions featuring much larger pixels, but the market demand high resolution photos. So because large sensor are much more expensive and result in a bulkier looking phones, phone manufacturers have no other option but to go with the market needs. I personally wouldn't mind having a 5MP sensor with a high-quality Stacked CMOS sensor with whopping pixels, but there aren't any phones with those type of sensors.
So you can now understand why an OIS on the Nokia 6 could have been such a great feature. I really don't know what the extra cost was to add this feature and how it would effect the overall design of the Nokia 6. Looking at other mid-range phones with OIS, I don't think it should have been a big problem to add this feature. Maybe Nokia is saving this OIS for it's upper mid-range and high-end phones only, I really don't know, but I can only assume that it is.
We also saw that some companies like Google with its Google Pixel was lacking OIS as well. I personally was quite disappointed when I saw that, and although EIS (Electronic Image Stabilization) works very well for stabilizing videos, it doesn't change the fact that an OIS is almost essential in mobile phone cameras when shooting in places where there isn't enough light to get a good exposure.
I have no doubt that with an OIS, the Nokia 6 rear-facing camera would have performed much better in low-light. I think that this is one of the features that just must be in mobile phones, no just in more expensive smartphones. When you see many low-light sample images on the web, you really don't know straight away what made that image look so good. Sometimes many phone reviewers credit it for the fast aperture, but behind the scene, the photographer might shot that photo with a very slow shutter speed and low ISO (whether in manual mode or the camera did it automatically in auto mode), and he didn't analyze the EXIF data to notice it or mention it in the review.
So I think it's definitely a disadvantage not having an OIS in phone from a company which is known for its great phone cameras. Just keep in mind that the video stabilization might work great, I am just talking about the OIS, not the EIS, if such exists.