It's not like there's anything to the romances beyond a couple different line or two and the silly "sex totally happened wink wink nudge nudge say no more" perk. I'm no big fan of Bioware's sexcuse romances but at least they try to weave it into the character rather than just having it as a side thought. It's still leaps better than Skyrim's "oh I am wearing this amulet let's get married" thing though.
As for companions in general I was pleasantly surprised. They're fairly one-dimensional but still interesting enough to not be flat, while there's plenty of tiny moments that do add to their characterization to stumble upon. Piper and Nick do have that annoying Bioware-esque "hey look how awesome this person is don't you love them" introduction/set up thing to them, but not quite as overt as Bioware does it and once past it they and the other characters don't really try to muscle in on your place in the narrative. Except literally, of course, but that's a pathfinding issue
So on the whole they struck a good balance where the companions are low key enough that they don't need any vast amount of depth to be acceptable, and there's enough around that if you do get bored by somebody there's usually someone rose not far away you can change them for. I do sometimes wish we had the chance to explore them more thoroughly, but another part of me feels if the price paid for more characterization would be worse writing then it's better to keep it at this level.
The improved quality of voice acting also helps build their character a lot compared to previous bethesda games.
Lastly I want to say that it is clear to me that Bethesda really wanted F4 to be more like Bioware or Obsidian games compared to their own older productions. This is both good and bad. I welcome the increased focus on NPCs and writing, but I'm also glad they didn't go the route of Bioware where lately it has felt as if their characters often try to steal the spotlight and like the writers often turns dialogue with them into monologues where the PC has little to no agency, as if that is what they think add depth to characters (characterization by rant, if you wish). The worst thing they got from Bioware is probably the crappy dialogue system, I really don't understand why they would do that. It's just so thoroughly crappy. Even worse is the possibility that the "only for options" rule might be hard coded. I really hope it's not, or that clever modders might be able to subvert the UI somehow if it is.