Both the iPhone X and the iPhone 8 Plus come with a dual cameras at the back of the phone. Now, the main difference between the two is that the iPhone X has a faster f/2.4 aperture telephoto lens compared to the iPhone 8 Plus that has a telephoto lens with an f/2.8 aperture. Again, this is just for the telephoto lens, the wide-angle lens is the same on both phones with an f/1.8 aperture.
The difference between f/2.4 and f/2.8 is 1/3-stop or 0.4448 stops to be more precise. This translates to a 1.361:1 exposure ratio. In other words, this means that an f/2.4 aperture allows 1.4 times (numbers rounded up) more light to pass through the lens compared to an f/2.8 aperture.
Of course we prefer having a faster aperture because we want as much light as possible to reach the sensor. This will lead to better low-light performance and better image quality in general, considering that all other hardware specs are the same.
This become even more important for telephoto lenses. The reason for this is that in order to obtain a sharp image, it's recommended to shoot at a shutter speed that is not less than the 1 divided by the focal length of the lens (in seconds). So the higher the focal length, the faster the shutter speed which is needed to obtain a sharp image.
Now, the the iPhone 8 Plus telephoto lens is 56mm equivalent, which means that in order for us to obtain a sharp image, we need to shoot at a shutter speed faster or equal to 1/56s. It wouldn't be an issue in daylight but in late afternoon, you'll find that using fast shutter speeds result in low exposure. A faster lens will allow us to get better low-light shots and it's especially important when shooting with a tele lens where there is a limit how low we can get before we start seeing a blurred image caused by a hand movement.
The good news is that the iPhone 8 Plus has dual OIS, so each of the lenses have an OIS system. This OIS system allows us to shoot under the recommended shutter speed (probably around 2-4 stops) and still obtain a sharp image.
The iPhone X does have an advantage here, having a brighter lens. In practice I don't think it will be a huge issue, but the thing is that even an f/2.4 lens is still not fast enough for low light considering the very small sensors found in cameraphones.
I would have liked to see a comparison between the iPhone 8 Plus rear telephoto camera and the iPhone X one to see the differences with my own eyes.
Overall, this is a disadvantage, but how severe it is, we still need to wait and see.