I'm comparing the Villa A sniper shots at the start with the Depot train shot here. When playing Villa, sometimes I could pump out both kills run after run (2 kills, run to basement, 2 kills, run to basement, etc). Other times I could go 5 minutes without killing both of them. It's really weird. My overall sniper success rate was less than 10% though.
Also, there were very few aborts mid-run. I either aborted at the start due to failed sniper shot, or at the end when I failed the basement. I'd continue the basement even if I knew I missed a guard.