>>723130820 (OP)
>Mass Effect
Depends where, I guess. They need the Mass Relays for FTL travel, right? (it's been years since I've touched ME). I'd say if it lands in any populated zone, then it's going to be a problem.
>Star Wars
If it's on the Outer Rim I could see it being a huge issue since those planets are more or less left unchecked. I feel like the tech in Star Wars would neutralize an infection form without much issue since you have so many droids in various different forms. Left unchecked and allowed to spread, it could be a big issue. Luckily, they have planet destroyers in this setting.
>Star Trek
Again, depends entirely where. If it's in the Alpha or Beta quadrant, it's going to be a big issue. Hard to say with the Gamma quadrant because of the Changelings. I don't think Flood could do anything to them. For the Delta, you have the Borg, and they would probably capture it and analyze it. If it can add anything to them, they'd try to assimilate it. If not, they'll destroy it. You also have literal realty warpers like the Q, the Douwds, and the Travelers (as well as the Department of Temporal Investigations who can rewrite the past if an anomaly shows up). If it gets too big, then one of those big heads will erase the outbreak from existence. If it's just the United Federation of Planets/Klingon Empire/Romulan Empire with no outside help, then they could potentially lose if the outbreak is massive enough.
>Warhammer 40k
Considering they're constantly dealing with Orks, Tyranids and other magical demons, I don't think the Flood would even register as a threat except to some outer colony. You'd probably have some cults rising up to worship it thinking it's connect to Nurgle.