How can it be difficult to develop when NMS and SC have it?
Adding more sky shaders? Probably not difficult at all.
Oceans.... probably not easy, but probably not rocket science either.
However, once you have those, then you have to develop the flight model for flying through different air densities and what are oceans without rivers, the changes to terrain atmosheres and oceans bring, weathering, and a whole boatload of other things?
Imagine if FD just added atmospheres and oceans without all the attending things they would require. I'm sure
certain forum members would start threads complaining about how poorly they were implemented. How the flight model doesn't change when you are in a thick atmosphere, that sort of thing.
How long would it take FD to do what is required to add real atmospheres and oceans? No idea... let's guess at 1 year. So, that would be Odyssey delayed for a year. You want Odyssey delayed a year? Or better they get Odyssey out when they plan, and deal with atmospheres and oceans later?