Chad breaks down the advantages and disadvantages of the user facility model, and why AMO physics is better off without it. I think he hit the high points, like the “economy of scale” advantage of having staff technicians to handle the equipment that isn’t part of your particular experiment, and disadvantages like
[I]f your small experiment that plans to use the particle beam from some accelerator or reactor breaks just before your scheduled beam time, well, try again in six months or a year, assuming they give you another block of time.
This was a constant fear when I was at TRIUMF, and something always broke just before we had beam time, requiring long hours to fix right before we went into “experiment running 24/7” mode, and had to surrender time on at least one occasion. We also tried to be ready to grab someone else’s beam time in case of problems, when the right target was in the beamline, and I have a vague recollection of doing that, as well.