Sebastiaan de With:

iPhone XS has a completely new camera. It’s not just a different sensor, but an entirely new approach to photography that is new to iOS. Since it leans so heavily on merging exposures and computational photography, images may look quite different from those you’ve taken in similar conditions on older iPhones.

But unlike previous cameras, exactly because many of its leaps in quality are based on software, we can expect it to change, and even improve. This is just the first version of iOS 12 and Smart HDR.

Likewise, us developers need to update apps to take full advantage of the new iPhone XS and XS Max’s very capable sensor. Since it is such a different animal, simply treating it as any other iPhone will yield subpar results. We’re almost done doing our first take at it and we’ll no doubt have to work more on it in the future.

Sebatiaan also notes a few specific RAW quirks with the XS:

As it stands today, if you shoot RAW with an iPhone XS, you need to go manual and under-expose. Otherwise you’ll end up with RAWs worse than Smart HDR JPEGs. All third-party camera apps are affected. Bizarrely, RAW files from the iPhone X are better than those from the iPhone XS.

I’m going to test Halide’s Smart RAW function, which should allow to extract the maximum quality out of the camera.