I don't think those are "large" assumptions, they're pretty standard things that can go wrong with every flight. Not that useful to have to enumerate them every time anyone talks about spaceflight.
- mission commander not being changed
- rocket not blowing up on the pad
- rocket reaching correct orbit
- mission not being completed, for instance docking issues
- everything that could possibly go wrong on the return flight
Really, the ease with which everybody seems to assume that this will go off without a hitch is impressive, this is not exactly walking to the corner store to pick up a small parcel.
Friend: I'm going to grab more drinks from the store
You: There are several very large assumptions implicit in that sentence
Friend: ?
You: Your car can break down, you can get in an accident on the way there, the store can be robbed and you can get shot, you may get a call about your parents dying on the way, a meteor may strike and kills us all...
Imagine if anytime anyone spoke about spaceflight, they saw fit to include a giant disclaimer of everything that could go wrong. We'd all just end up sounding like drug commercials.
* You could sprain your ankle and not make it
* The parcel might have been stolen by a robber
* The corner store might be on fire
* Feral dogs might chase you to the other side of town