As a Roblox developer, it is currently too hard to make immersive & dynamic environments where players can go to new regions, where said regions have differing atmosphere properties. When tweening or setting Atmosphere properties every RenderStepped step, Atmosphere settings do not instantly set; instead, chunks of the screen change over the span of several frames, presumably to minimize lag. A side effect of this means that trying to tween over two Atmosphere values will simply look bad and choppy, which means the only way to go about it would be to instantly set the values instead - which is not UX-friendly nor intuitive.
Since it was implemented for lag-mitigating reasons, it should only be available for higher graphics levels.
The reason this is a feature request rather than a bug is because a Roblox staff member has told me this is expected behavior; thus, I would have to make a feature request instead, which I now have.