One issue with defensive metrics - and any other subjective defensive evaluation technique - is they rely on opportunity. If a player makes fewer plays, he isn’t as valuable defensively.
A really big factor in Bradley’s defensive metrics this year is he hasn’t made as many plays as most MLB center fielders. And a big part of the reason appears to be Betts, who helps take a huge chunk of right-center field flyballs that Austin Meadows and Avisail Garcia don’t get to (which is why Kiermaier makes more plays).
This isn’t a flaw in defensive metrics. Eye test can’t tell you how good a CF is on plays he doesn’t make either. But instead is it brings in an extra level of evaluation. For example, as long as the Sox have Betts, do they really need a CF with Bradley’s defensive skill set? If they moved Benintendi to CF or acquired a less range-oriented CF like Joc Pederson, would there be a significant difference? Sure they would miss out on Bradley’s highlight reel plays, but what is the actual big picture impact on defense?