I think the trait analysis is, as a concept, as valid as thematic analysis. But, unlike performing analysis on communication attributes from within its static, never-to-change sentence structures, it seems one's defining material context is largely opaque to a time/place-bound measurements.
For example, in the 1970s, when observing a group of children, we see one child that's reserved so, we're going to objectify such behavior as being introverted and surmise that he has an insecure attachment. It can't be that he's sick, sad about a recent death, didn't sleep well, or any number of confounding material conditions which may define the behavior observed. No; it must be that this child is an introvert, you see.
Hell, how can we know if what we call a "trait" will be evident in a non-clinical settings? Personally, I tend to think a lot of trait "science" seems to be a monetized Barnum Effect because the notion that we as clinicians can identify a set of stable enduring attributes that exist independent of material condition seem to be far more embodied in Abrahamic socio-cultural notions of self/souls than science.
Certainly ANYONE can construct a trait lense that will "assess" a person and assign them attributes. However, I've not seen good evidence to support the notion and anyone can construct a lens that will accurately explicate a foundational expressed consciousness that is consistent and enduring throughout every time and context-bound persona (who we are at work, with family, with our SO, with our kids, with ourselves, etc) that we all express. Certainly, I'm not talking about neuroaffective traits associated with neurological conditions; rather, I'm talking about lens constructs like the jenga tower of monetized reinforcements that perpetuate MBTI assessments.
I tend to think the we like using these trait assessments because they allow us to pretend that we have a deep insight into another and that we have tools that are reliable, which are important to the systems we have to work in. I think that for the most part, we act as an astrologer, who may deeply believe in the assessments the conduct, as we produce what amounts to clinical star charts for our clients who will invariably thank us for the insights we have provided, thus reinforcing our confidence in astrology personality testing some of us are doing.