This being my first Fallout game play through, I can't explain why the devs used such lower quality textures, but one thing I know Shadows of Modor was like 40+ GBs while Fallout4 was something like 22+GBs. They clearly went with lower rez textures. That's on them to explain. (The texture quality also isn't THAT bad, most likely because I play a lot of MMOs haha).
There are tons of games that use horrific texture quality, and again - that's on them. I don't know their budget, their art system, or why they chose the texture quality (even for "Ultra") that they did, all I know is for me that is the last thing I sit and stare at. I actually find it if the story and gameplay is not engaging my attention I start to criticize graphics.
I'm pretty sure Fallout4 falls into that category of highly modded game. My play through of Skyrim was years after it came out and I had so many mods to pick from. I don't even know what the base game looks like to be honest haha.
The performance hit from Ultra for Godrays is definitely high. You can run the game on High and not really notice an IQ difference but you will notice better performance. I just trumped that up to the e-peen phenomenon. Sometimes "Ultra" is just there as a "because we can."