For that matter, those Starships themselves are incredibly dangerous machines. I remember reading somewhere that only one of the original twelve Constitution Class Starships returned from their 5-year missions in the mid-23rd century. Yet starfleet insists on continuing to not only build them but to enlist thousands of innocent sentients every year to crew these death-traps. The only Constitution that returned from its 5-year mission tragically lost nearly a quarter of her crew under the command of her most famous captain.
Just sayin.
Truth to be told though We don't know that the ship itself was at fault for most of those Constitutions not returning. Is it even canon that only one returned?
One of the old novels might have mentioned such statistics on missions (I can't think of where) but nothing on screen that I can recall. Even so, holodecks and starships are completely different machines with different purposes.