I think the major complaint there is that the setup in previous Fallout games was a little more open. You had a goal and a vague back story, but never something so life defining as "You're married in a heterosexual relationship and you have a child you care about, if you're male you were a soldier and if you're female you were a lawyer.". Previously it was just a case of "You grew up in a vault/small village/your dad is Liam Neeson, go"
As much as I loved Fallout 4, it definitely breaks your own sense of immersion if you're the type to roleplay your own characrer. It hamstrings what you can/can't invent as parts of your own backstory, and while you can ignore it for the vast majority of the game - every time you hit a main story quest your character has a whiplash moment of character regression where they suddenly turn into Story Main Character and react totally differently to everything they've seen/done up till that point.
The narrative flow of you, the protagonist, completely breaks down every time a main story quest starts and it only gets worse the longer you wait to do it. Its like they designed the main story to be shotgunned immediately upon leaving the vault under the assumption that everyone would mainline that part of the game for 5-6 hours to watch their character progress as the Man/Woman Out of Time, and then go back to the open world afterwards where they're nonchallant and well adjusted to their surroundings, but obviously very few people play the game that way so instead you end up with a character arc that just kind of is totally scattershot.
Took the words right out of my mouth.