With Daggerheart, since more of the storytelling and worldbuilding are offloaded to the players, does it run the risk of suffering from "design by committee," where the resulting mix of styles and plotlines creates a game that lacks a cohesive vision?
I would say yes but no or perhaps yes in a good way.
You lose the cohesive vision but instead gain a consensus vision which, in a small group format, can be a net gain.
I feel like Daggerheart run "as intended" should have between 3 to 5 players, and 5 is pushing it. For me this was one of the major flaws of watching Critical Role play it. Their cast is too large for their own game. As frustrated as I am that CR picked D&D over Daggerheart, I would never want to run or play in a Daggerheart West Marches game. I feel you would lose what for me is the main reason to pick Daggerheart - 'Description on demand's ability to heavily influence the narrative going forward.
To do that you need a group that very rapidly starts to vibe off each other and round robin the roleplay almost as if by instinct. And that means a small group.
The perfect example of Daggerheart working at it's best is the Dodoborne podcast. If you can get past the comedy and the one player who talks in a funny voice (why do some players who are able-voiced do that... I almost stopped watching Dodoborne over it)... anyway, get past those two things and instead pay attention to HOW they play the game. And how the players will just go on for long stretches roleplaying while the GM is just completely passive, almost silent. The GM will just pop in an odd 'sure' or 'yeah' every so often and the players just run with the story at times.
There are three of them, and they are IMO the 'example case' of 'Description on Demand' when it works to perfection. I doubt most groups could pull it off. But these 3 do it so well I sit there wondering how they are not a bigger 'thing' than they are. Sure they have a rapidly growing fan base, but I can't imagine they won't keep growing.
So that game is more or less "design by group vibing to a 'this is fun' consensus".
And you'd be very hard pressed to get that success if they added even 2 more players. They might be able to get to 4 players if they got extremely lucky. They'd have 'State Lottery odds' of keeping that energy going at 5. And past that and I'd be betting on 'planet killing asteroid' over it still working.
Which you can kind of see when you watch Daggerheart actual plays with larger casts. They end up feeling like D&D games - at least to me. They might be good, but they lose what makes Daggerheart special.
So yeah. There is 'design by committee' in all of this. But when your committee stays small enough that's a good thing.
As an aside - ever since CR made their season 4 announcement the DH community seems to filling up with people planning West Marches games and I'm starting to feel none of those games will manage to capture the 'Daggerheart magic' because it's just too many people to keep the flow going right.