There are certainly flaws in UZR/150 and WAR, but to me the flaw with personal comparative player skill analysis and the "eye test" is greater (maybe with some players or positions more than others).
It's weird that Swihart has 6 innings on defense and has a number. JBJ has 15 innings in RF and it is blank.
Also, I'd have categorized the catch he made in RF the other day as "impossible" not "remote", so I'm sure subjectivity plays a role in their categorizing play difficulty through "eye test" observations. After all, it seems like UZR/150 is just a massive amount of "eye test" observational scores entered into a computer program.
I can't and don't watch hardly any other baseball but Sox games. There's no way I could ever say for sure, "JBJ is the best defensive CF'er in MLB, today!"
UZR/150 and DRS tries to quantify eye test observations from trained observers who do watch every play of every game. I think the idea is a good one, but it does have flaws. Maybe someday, there will be a way to objectively rate and rank defense. It's not quite there, yet.