I haven’t played KSA yet, forgot that it was a thing, but one of the big ones is a custom engine as opposed to the cobbled together mess of code that is KSP, which is running on an engine that is absolutely not meant to do what KSP does. All the weird physics glitches in KSP are because it’s trying to wrangle the engine into functioning in this way.

KSP2 was actually supposed to fix this as well with a brand new engine, but the publisher forced them to use KSP’s engine “because it would be faster” (it wasn’t).