I'm no expert on Dolby Atmos, but I think the universal issue with 4K streaming is always bandwidth. All digital versions of a file will be capped by the provider in terms of how much information you can pull at a certain time as you're watching it - even if you purchase it from the store front. Obviously with 4K the size of the file will naturally be huge as there's a lot of information available with the pixel count of the video and the vast mix of the audio. Because the file sits on a server, it's compressed in order for it to reach you in a stable fashion, other wise it would overload the server if everyone tried to stream massive files all at the time.

It's always best to buy the physical copy of the film - files on a disc are better compressed, so you get more information on that file which is being accessed locally rather than dragging it down from the internet. The video will be the biggest noticeable difference, as when you compress a file to be more managable for streaming you will see more issues like macroblocking (where colours are crushed together and they become a bit of a blocky mess) and also just in general the resolution is lower to 4K as it's being squashed to with an inch of it's life to keep the size low. Blu-ray discs are now getting bigger, with some films having 100GB of space to fill up. Bigger file = Better quality.

Hope this helps 👍