There are a lot of both dark themes and on screen deaths and violence, many of which are pretty graphic.
Fallout as a franchise is well known for some pretty horrific elements, often painted over with bright colors and upbeat music, but horrific all the same.
If Game of Thrones or The Boys were too much for you, then Fallout certianly is.
That said, it is an absolutely brilliant and faithful adaptation of the source material, and as a long time fan of the games, I loved every minute of it.