Well ... expert rock climbers routinely trust their lives to gear which is not their own, namely slings + chains + rings for abseil/rappel. And sport climbers routinely risk their bodies to trusting that the first bolt off the ground will hold a fall.
A key issue is if it is straightforward to inspect a piece of gear whose history you cannot know (which climbers often feel is true of equipment for abseil/rappel) - (but is generally not true of sport-climbing bolts).
Another factor is how “over-engineered” a piece of gear is for its use. And of course the reputation of those who manufactured and installed it.
Via Ferrata kits used to be simple and readily inspectable: e.g. the Petzl Zyper—I doubt you’d have much hesitation about using a rental Zyper if you could inspect it before leaving the shop, and you were trying your first VF route and it was of great reputation and you had every reason to believe it was well within your ability.
Then recently two major innovations: (1) “screamer” style shock absorbers replacing “rope thru holes in a plate”; (2) elasticized lanyard arms replacing simple nylon slings or rope. The second is just convenience, the first arguably improved safety especially for lighter climbers. But both make inspection by even an experienced user much more difficult.
Add to that the tremendous growth in popularity of VF drew many new manufacturers and distributors into the game—each trying to make some claim to new design feature, or lightness, or cheapness. With each new model and feature a learning curve in manufacturing, especially when the brilliant design engineers are in Europe and the factory is in China.
Add to that that many VF routes have their cable anchors positioned for best Aid, not for protection in case of a Fall, and that many VF climbers are even more clueless than novice rock climbers, and . . .
the surprising thing is that there have not been more equipment failures.