So I've now got some experience of all the factions, and I just feel thoroughly disengaged from this game. I literally struggle to play it for more than an hour or two, an experience which I've never had playing a game before and I find incredibly surprising. I'll break down what I think is bad into a number of points:
1) The dialogue for the most part svcks. There's far more people in this game than ever before trying to engage you in dialogue and tell you about the troubles they're having/something about their shop/what they had for breakfast/whatever - and it just leads to this bombardment of dull rubbish non-stop throughout the game. More often than not, your character also has very little to say to these people in return (3/4 dialogue choices amounting to the same thing), before it's back to the same dullard windbag to tell you more of their problems.
Regardless of whether I like/dislike the choice to go for a fully voice acted character, they're really not used enough in dialogue exchanges - with masses of time being taken up with people talking at you rather than you having any genuine input at all. The writing of the dialogue just feels off in general - it lacks any kind of semblance to real dialogue whilst at the same time lacks the wit/pacing/interesting exposition of video game/movie dialogue. Bethesda really need some better dialogue writers.
So, first negative point: Talking to people is dull and unrewarding.
2) The factions are either preachy or unrelatable, and becoming invested in one of them is mandatory to further the main plot. Lets take them one by one. Minute Men - pretty much just looking to help people (relatable), but Preston really makes them super-preachy. BoS - want to fight the Institute because they have too much dangerous science (unrelatable - though at least it ties to the lore), and also super-duper preachy at the same time. Railroad - deadset on helping synths for some reason (unrelatable), and also super-duper preachy. Institute - want to help and better mankind through science (relatable), but also have a really weird bunker-mentality going on that seems counter-intuitive to implementing what they want to do practically (unrelatable), and they are slightly preachy - albeit less so than the other factions as they seem to think they're superior already and so don't feel the need to explain themselves.
They're all pretty much populated by ideologists who really believe in the cause, without any real trace of internal corruption or intrigue. The game is incredibly faction orientated, and while random exploring can be fun for a while you're always going to be pulled back into their games. The vast majority of factions are pretty damn unrelatable, or they are really asking you to care way too much about what they do, to be fun. This, rather than enhancing sandbox gameplay, is rather restrictive of it - as the political situation is so much at the forefront that it's very hard to ignore.
Second negative point: Overbearing preachy factions which permeate the entire setting expect me to care when I just want to be doing what I want and having fun.
3) Your character is very much not a blank slate, there's always the son to be running after. If you're careful enough you can avoid mentioning the son for a good long time, however the game always funnels you back to seeking him out. The fact that this is such an integral part of the story and that it hashes out the same every time makes it very boring to repeat, and an irritating niggle if you want to play a character who didn't care about his little scamp.
Third negative point: You have to proceed very linearly through a very big part of the story, and in doing so you also have motivations ascribed to your character that extremely presumptious in terms of what we - the player - would want our character to believe in and care about. These guide rails also impinge on the sandbox gameplay.
For me a sandbox is just about the freedom to explore, it's also about the freedom to play a character that I want to play. Point one means that when I'm looking for reasons to explore, I'm finding very little interesting reasons to go places because the dialogue for the most part isn't that sharp. Point two means, when all my exploration has been done (which it is now for my character), I have to get into bed with these factions that are either unrelatable or have an unrealistic expectation of how much they want my character to care about their goals - the latter of which makes my participation in what they're trying to achieve seem false. The third point means that there's this constant pull on my character to do something and involve themselves in the story, making me play a character who has possibly has different beliefs to what I would want them to have but also encouraging me to not have such a riotous time doing whatever I want in a sandbox game.
Boiling it down: Boring dialogue produces dull side quests that provide little incentive to explore the environment. Though exploring the environment without side quests is in some ways its own reward, it only lasts so long, then you have the game telling you that you want to find your son and then that you should want to get on board with one of these weird ideologically strange factions. It's all just a massive disengaging turn off, producing turds in a sandbox game - i.e. limiting or discouraging your own free choices and removing impetus to explore in a game that's supposed to be exactly about that.
How Skyrim did it better: The Dovahkiin was a true blank slate, and though the game occasionally prodded at certain outcomes (go to Whiterun, etc.) the motivating factor behind these outcomes was no where near as large as the drive to save your own flesh and blood which you care about, and there was so much to do aside from main quest/faction related stuff it was easy to get lost doing other interesting things. Though the dialogue was similarly svcky, it was at least short and was used to emphasise exploration. The Stormcloacks and the Imperials had somewhat relatable goals (seemed very anologous to the American War of Independence), but best of all you didn't even have to ever engage with them if you didn't like their whole deal. All in all, a very strong emphasis was here on freedom of exploration and doing whatever the hell you pleased because the game wasn't telling you what kind of character you were playing and it wasn't telling you that you needed to care and join with a certain political faction to further the game.
How Fallout 3 did it better: In Fallout 3 your Dad, though still a relative, is a grown man - capable of taking care of himself. Besides, he abandoned you and made a crapstorm by leaving, your character very may well not feel like tracking him down very hastily. Exploration in the game was great, and dialogue was kept to the point/expositional - not seemingly endless guff. You're made to side with a faction, but in the end can betray them, and their goals are straightforward and relatable at the very least (trying to help people). Your character, in his desire to track down his father, really doesn't convey the same emotion about it as is present in FO4 and with your upbringing in the Vault being however you want it to be there's really no imposition on the kind of character you're playing. Just like Skyrim, a great sandbox game.
How Fallout: New Vegas did it better: The game, while faction heavy, provides factions that are relatable. Legion and NCR are trying to make the world a better place by imposing their own style of governance, and House has pretty much the same goals as the Institute (rightfully so, given his background) but doesn't have the same ridiculous bunker mentality they do and is engaging with the outside world in a way that engenders trust in his leadership. Ultimately, if you don't like the factions, there's always Yes Man which provides a nice way to stick it to all three. Factions are also populated with normal people who care about their own faction to differing degrees, and have corruption present, making them inherently less preachy then their FO4 counterparts. The character is again a blank slate, and the exploration, whilst worse than Skyrim/FO3/4 is still pretty decent. I think the implementation of factions with normal goals (install democracy), compared to pie in the sky goals (free the synths), and the fact there's all those human elements to the factions on top of you can do an ending completely independent of the factions really makes FO:NV a much better faction orientated game.
I honestly don't get where Bethesda went wrong here. The main reason I'm posting this is I want Elder Scrolls 6 to be actually fun to play for several hundreds of hours, not the bare 50 I'm struggling with now.