There could be side-effects we don't know of yet for either option, like pissing off a NPC by effectively blockading them, or in spite of the 'no repercussions' aspect having income damage by failing to enact the quarantine and some ship explodes and we all get hit for cash for 'emergency relief'.

Those possibilities aside, being ethereal, obviously nobody with assets there, economic or military, would be in favor of this. It doesn't say how long the quarantine would last, one round, multiple rounds, or the whole game.

Now if I understand correctly, everyone started with 7 attributes points and 7 assets from them. Diplomacy points got you core worlds, military fringe worlds, and intelligence outer colonies. Brontes, the world to be quarantined, is one of the 6 core worlds and thus anyone with a D of 1 has a 1 in 6 to have an asset there, D2 would be 2 in 6 etc. So people with the highest diplomacy scores, and most votes, are most likely to have assets there, so even though an assumed average D score of 7/3 or 2.333 would indicate a slim majority didn't have something on Brontes but odds are the people who do have more voting power at the moment. The mod might be able to clarify that, I'm not exactly sure how much vote we all have and am sort of guessing.

In any event, if you've got territory there you obviously vote against it, if you don't then it is probably better not to but one does have to worry about revenge or retaliation, especially if the quarantine is more than a round long. Direct military retaliation is improbable I'd think but as those most likely to be effected are those with higher diplomacy scores and more votes future effects in the fringe or outer worlds are probably more likely to be voted against by those with high diplomacy scores as retaliation.

Other than that the obvious thing to remember is if you do have territory on Brontes it would probably be unwise to assign troops there from your floating garrison. Unless you have a decent military value and can get them there before the quarantine starts and think you can find someone else's assets there, in which case you might be able to swallow up assets while people can't reinforce them.