Like I said, I can buy a small-scale attack on a ship or small starbase from a sphere, or a scout like we saw at the beginning of "Dark Frontier," not being something we'd hear about. But a cube attack is so large and so inherently dangerous that I just find it implausible to imagine that one had happened.
And like I said, this isn't a coherent reality, it's a bunch of works of fiction that pretend to represent a coherent reality but sometimes choose to portray it selectively. There are cases where big events that you'd realistically expect to be mentioned do not get mentioned because the realism of a work of fiction only goes so far. Stories need to concentrate on events that are directly relevant to them, and so they don't generally include mention of outside events that don't directly pertain to the story, no matter how big they are.
Heck, this even happens in the literature. The events of
The Genesis Wave were portrayed therein as extremely cataclysmic and affecting many worlds and many billions of people. And there have been enough references to elements of TGW to establish that it did occur in the main novel continuity (or at least the first two books did, since the third has elements contradicted by
Titan). However, that reputedly massive cataclysm is rarely acknowledged in other books, its effects never really shown.
And as I said, apparently the Federation was at war with Cardassia during TNG's first two seasons, but those seasons portrayed the UFP as being entirely at peace. "The Wounded" retconned a conflict into existence that had never been hinted at before. So these things do have canonical precedent.
But these can be dealt with by taking a cue from Poul Anderson's Terran Empire series, and realizing how difficult it would be to keep track of events within a civilization spanning hundreds of entire star systems. Most of us don't know a fraction of what's going on right here on this one planet, or at least aren't directly affected by it. For instance, right now there's a massive drought and famine in the Horn of Africa, governments are being overthrown all over the Muslim world, the Greek economy is falling apart, etc. -- but if you were to write a story about the biggest thing that happened to you or the people around you in the past month, there might not be a single mention of any of those things, because they didn't affect the events of your particular story. And when you expand that to a Federation including hundreds of worlds, it becomes even more inevitable that you could have a massive, cataclysmic event in one part of the Federation and yet hear virtually nothing about it in another. Sure, it's a bit of a contrived fix for continuity problems, but it's a plausible one, and those problems are going to happen no matter what.
Which makes a small-scale attack plausible. Didn't No Limits feature a short story about a Borg sphere attacking the Excalibur?
I don't recall if it was a sphere or a cube, but yes. However, that story got the chronology wrong, since the story is set in 2369, a year after the flashbacks in "Survival Instinct" featuring the assimilated
Excalibur crewmember Marikah Wilkarah.
