Yes, exactly. I read the description and think, "Oooh...shiny! I love that concept." Then I read the abilities and think, "Hmm, ok. Is that it?"
Yup, I'm exactly the same. I often like the flavor but find the implementation lacking, mainly because, having played the game a lot, I know that most characters won't get much past 5th to 8th level and, thus, will only ever get one or two of the listed features. Given that WotC has said that, according to their own surveys, groups capping at around the 6th level mark is really common, I have to assume that my experience is unexceptional. Add this to the fact that the first feature you get - sometimes, the only one you will ever get - varies wildly in terms of "flavor impact" between different archetypes (i.e. some are fairly heavily flavored, others much less so).
The problem is that, to really fix this, you need to have more "insertion points" in each class, to slot in the unique features (presumably, at the expense of standardized class features, if you don't want to change the power level). I've been puzzling over the least messy/work-intensive way to do this, but, to be honest, haven't come up with a really satisfying solution. Maybe PF did it best by having each archetype slot in, via replacement, only as many or as few changes are needed for each individual case. Harder to balance, but, ultimately, more effective in making each archetype as distinct as it needs to be.
'Course, in terms of UA archetypes, WotC has made the situation worse by sometimes trying to turn what used to be a full class into an archetype. Because of the low impact the archetype distinction has, due to the limited number of insertion points, the result inevitably feels watered down. But that's probably getting a bit beside the point, given that it could be argued that some of these shouldn't be archetypes, in the first place.
Anyway, having said all of this, I'm a bit uncomfortable with the feat idea, for several reasons. First, some groups don't use them, so, for them, that's kind of a non-starter. Second, you get feats so rarely that the impact may not be that noticeable, especially for players who want to get any ability score points. Third, because of the way 5th handles feats, finding a nice balance might be tricky. In previous versions (and in PF), there was a very rough correspondence between feats and class features, but, in 5th, that's not really true (feats are generally somewhat better). Now, in this case, I suppose that might be a good thing, since it could give the archetype a particularly noteworthy class feature, but, again, finding the right balance might be tricky. Possible, but tricky.
Pondering the last thought, maybe the feat could gain it's potency as much through synergy as through intrinsic power? In other words, the feat could do something that, in most hands would just be moderately useful, but, when paired with a particular class feature, would be more interesting/useful/flashy. Something that makes the archetype more distinct. Of course, none of this addresses the first two problems that I mentioned above.