The crpg Morrowind is a sandbox, a very large one, but also has a main plot featuring, imo, at least one brilliantly jaw-dropping reveal.
In the same way I can see how a pnp sandbox could contain one or more adventure paths, reaping the benefits of build-up and more meaningful climax that that brings. I understand some people run APs with elements of sandbox, allowing a certain amount of deviation from the path.
I've only played a little of Morrowwind, but I've put in 200+ hours on Oblivion. Great game.
However, one aspect I don't like about Oblivion (which also seems true in Morrowind), is that the PC can pick up a ton of quests and the passage of time does not affect them (for the most part).
The initial problem is that many of these hooks should have a consequence for too much time passing. In turn, this would cause the side effect of with all these problems happening effectively at once, the PC is not able to solve them all. This in turn would lead to the bad guys winning a majority of these quests, by virtue of the world only having 1 character who actively solves quests.
This gist if you have too many simultaneous evil plots that can only be solved by the PCs, then evil will win a majority of those plots and dramatically change the campaign world.
Part of the solution is that for any quest the PC doesn't undertake, an NPC hero MIGHT do it, thereby preventing disaster.
Another part of the solution is to only present a limited set of available quests any given time. In Oblivion, once the PC has 3-4 quests, don't present him with any more (assuming the PC can accept a quest or refuse to add it to his list). Thus, there's only a few quests that could "time out" as opposed to all of them.
All of this is supposing that too many quests that are in danger of timing out (i.e. the bad guys win) is a bad thing. I would argue that unless you want to dramatically change your game world to a state where the bad guys now have the upper hand, that this would be a problem then.
Now one side argument I think I saw from Hussar, was the assumption that a sandbox could entice 6-7 players more than an adventure path style game. I suspect that's not the problem at all.
If you have 4-5 players, it is easier to get them to reach a consensus on what to do next than a party of 6-7 players. Regardless of play style. The more people you have the more different directions folks are going to tend to want to go.
You're either going to split the party or they'll be timeswapping between goals they're pursuing. I don't think the problem is exclusive to either style.
Having run a game in my style with 6-7 players before, what helped me was setting the starting state of the PCs. When I did it, the initial state of the campaign was that the PCs were members of the navy or marines serving on the Sea Sprite. I told the players the way the campaign was going to start (y'all are in the military, serving on this ship). They then made characters that would fit in this starting state, and we played.
The part I cheated on, was having them be military, they effectively worked for somebody and took orders (went on quests), solving some plot hook buy-in. It would be easy to stay in a rut, but my goal was to take that framework and have the PCs pursue more personal goals and challenge their loyalty, to see what they'd do.
My core point though, is if you have a party of unrelated people, you'll have a hard time getting them to agree to do anything. This means the game will bog down. There is value in setting some parameters of character creation to get them pulling together so the group can have a game. And if you've got 6-7 players, you can easily afford to lose a few who don't want what you're running.