Nuclear tends to have a higher capacity factor than most other baseload generators, but it also has unplanned outages.
https://www.eia.gov/todayinenergy/detail.php?id=45176
The biggest risk I've seen is that unanticipated events (including financial events) will completely shutter a unit, like San Onofre and Indian Point 2.
https://en.wikipedia.org/wiki/San_Onofre_Nuclear_Generating_...