If the goal is to regulate unhealthy foods then it does kind of have to be perfect (very low false-positives). UPFs as they are defined include baby formula, many frozen meals regardless of macros, soy/almond milk, instant oatmeal, pasta sauce, flavored yogurt, etc.

Invariably when someone says something like "UPF is a pretty reliable heuristic" its because they are massively underestimating what counts as UPF and using a "I know it when I see it" approach, which, yeah of course it seems reliable if you start with the precondition that UPFs are unhealthy.

If it's just guidance and not for regulation, well, you have similar problems in the opposite direction. prepackaged whole grain bread is UPF the same as Wonder Bread w/ 2.5g added sugar per slice. It's easy to say "just buy fresh bread" but when that collides with the reality of a busy schedule then UPF designations become next to useless. The undeniable value that preservatives have for healthy and unhealthy products alike mean that anyone using actual UPF as their heuristic will be completely rudderless.