https://www.reddit.com/r/buildapc/comments/3scaku/why_fallout_4_is_running_so_bad_on_your_amd_card/
Bethesda jacked up Tessellation to excessive levels, similar to CDPR and Hairworks. Why?
Because Nvidia can show off their geometry engines. Simple.
God rays on low or medium should run fine on AMD.
Let's not forget that the performance issues on AMD cards can't be pined down on Nvidia, it is a known fact that AMD drivers are just not on the same level when a game stresses the CPU heavily.