The problem is not graphics, but rather, the limitations of Unity that make it unsuitable for anything else but lowest common denominator multi-platform games with clearly defined limits. Space Engineers is another game hampered by Unity.
Unity can do far better graphics than KSP has e.g. https://unity3d.com/pages/adam. The fact that there are numerous mods which vastly improve the graphics of KSP is pretty good evidence of this!
Where Unity can be blamed for KSP problems is the constant stutters due to GC - Unity uses an ancient garbage collector. Hopefully that's going to get better soon - apparently there was some licensing problem keeping Unity on an ancient version of Mono, now that MS own Xamarin that's probably not a problem.
Essentially, most (unity) games don't allow their users to create content, whereas the essential part of KSP is the creation and simulation of lots of bodies.
I've definitely experienced a substantial slowdown when many entities are on the screen at the same time.
I'm not sure if KSP makes use of the normal Unity physics engine, a third-party one, or something developed in-house.
[0]http://forum.kerbalspaceprogram.com/index.php?/topic/139128-...
I mean, this is how my modded KSP looked back in time of 0.25C: https://i.imgur.com/yBahDMr.png. I could name AAA space sims that look worse (esp. in terms of immersion).
Say what you will about No Man's Sky, but that game was gorgeous, which contributed in no small part to its widespread interest. Now imagine how fantastic a game with those graphics but with KSP's mechanics would be.
Sit me down in a room of 20 games, and tell me to identify unity games by simply playing each computer. On the 20% chance that I can't instantly tell by looking, a wee swipe of the mouse and the complete lack of responsiveness finalizes all I need for 100% success rate.
That said, it's not easy though it is certainly doable; it basically means you're using Unity largely as a "rendering pipeline" which may or may not be overkill, depending on the use case/game in question.
You're right -- there's very little written on the topic. Though Unity has a fantastic community, and a large swath of information & user help to be found, a lot of it is directed at the newer/simpler developer (or mobile specifically).
Much of our work, and findings, has been derived from running our own tests & benchmarks as well as reading research/white-papers. In general, we've kind of found that many [advanced] game dev related topics are not well publicized or are fairly tightly-held. Our best source of information for things like highly advanced pathfinding, or low-level & high-volume multi-agent modeling/simulation strategies, have come directly from published academic research which has also been a treasure-trove of information.
We've read more research the last year or two than we did through all of school and the first ~10-15 years of our careers combined! =)
I guess most of the physic slowdown is some improvised custom code around joints that makes the solver choke
The very act of double clicking the icon to boot up unity === terrible performance.
"Maybe she's born with it" was a funny joke I recall when working in Dundee on a Unity Project. I remember moving to a temporary role (More to fill the team for the investors to smile) and they were using Lumberyard and what a total difference.
Unity is a poor mans game development solution and that might have perks, but a game the scope of KSP deserves way way way way more respect.
Well-crafted art assets are far more important to the look of a game than the specific engine that is used. The reason some "lesser" platforms like WebGL or the various open source game engines look so bad is because they are far more accessible to beginners, beginners who don't have access to good assets.