I suspect the mechanical issues can be made redundant enough that any long, say a year, mission has a very good chance of succeeding. If critical functions can be made simple that would help.

I'd be more worried about the wear and tear on folks psyches. In the relative isolation and confinement of a long-term mission rather subtle traits may become much more pronounced.

I also suspect there are any number of pathologies for which there is little or no screening. The Navy Captain who went south over the boyfriend is one which comes to mind, and the boyfriend by running around with a married colleague demonstrated somewhat poor interpersonal judgement. What might pass on the ground, or near it, wouldn't necessarily be wise on a year-long trip.

I seem to recall some ISS folks thinking Ground Control didn't care about them and their difficulties, and getting resentful. And they didn't have a lot of time to sit around and think.

Most human minds are in very bad company when alone for an extended period. And that goes for even relatively alone.

It will be an interesting challenge.

Arf