Is a BRAND NEW 1996 Subaru Outback Thermostat infallible

Probably not.

Absolutely not.. nothing is. New parts nowadays can be just as much failure-prone as something that is been in service for ten years. Is there a particular issue you're having that a new thermostat failed to fix?

Be aware these engines were prone to headgasket failures due to a poorly designed gasket. Until this problem was nailed down, a lot of shops would simply throw a T-stat at the car, hoping to cure the problem, & when it failed to do so, they'd simply chalk it up to a defective 'NEW' part.