These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Hollywood Science: How hard Orion's Arm's "hard sci-fi" actually is is debatable. The project has many critics who attack it for unrealistic technological optimism, especially in fields like biology (the genetic code of complex organisms is extremely difficult to successfully re-engineer to the point where Artificial Humans may be more practical than enhancing real humans), and its implementation of transhumanism and "Singularity" (which has many skeptics and makes a lot of assumptions about the nature of intelligence and computers). The members of stardestroyer.net have produced some especially comprehensive and hostile criticisms.
Then again, these are the same guys who treat Star Wars as hard science fiction. While they may (and in fact, do) have valid points, it seems a bit unfair to hold Orion's Arm (and Star Trek among their other targets) to a higher standard.
It is also important to note that the term "hard science fiction" is hotly contested even outside the context of Orion's Arm. What one person considers to be realistic another laughs off as either foolish optimism, in one direction, or not thinking big enough, in the other. At best, hard sci-fi is a broad blanket term for anything that conforms to known natural laws as we currently understand them. With this, Orion's Arm can definitely said to be hard in terms of physics, if a bit looser in terms of biology and computing.
Inferred Holocaust: The Terragen Sphere and other civilizations continuously find the remains of prior civilizations, some of which spanned massive distances before simply disappearing. Something keeps wiping out all those galactic empires. Naturally this has gotten quite a few scientists worried about how long it will be before said mysterious force shows up to put down the Terragen Sphere...