I agree with all of that, except one part. The offensive numbers also are skewed by things out of control of the hitter, but people seem to dismiss luck and other things on d, without acknowledging the luck in o.
But we agree completely that UZR is not precise. I think everyone that is serious about stats agrees with that.
To me, the problem with comparing offense and defense and attributing it to "luck" is this:
While it most certainly is true to say that (to pick a name at random) Pedro Florimon was "lucky" to string together a good month and hit .400, it is not at all true to say it didn't really happen. No matter now lucky it was, we know for a fact he hit .400 and the value of that cannot be disputed. He had 100 ABs, got 40 hits, and hit .400.
Nobody contends he didn't really hit .400. It represents exactly what happened on the field.
Compare that with what the designer's of UZR say...a good or bad UZR doesn't mean that's what happened:
...a player’s UZR, be it one year, one month or 5 years, is not necessarily what happened on the field and is not necessarily that player’s true talent level over that period of time either. That is why we regress, regress, and regress! A player can have a plus UZR and have played terrible defense, because the data we are using is far from perfect.
If it doesn't represent what happened on the field, I have a hard time giving it any credit. If we're going on opinions, I'll go with the opinions of professional baseball people, or myself, rather than MGL.
You can't have a .400 BA and have hit poorly. Luckily maybe, but not poorly. Those hits actually happened.
But if you can have a plus UZR, and "played terrible defense," the measurement system needs work.
EDIT (meant to include this with the original post): As for regression, I'd say if you're "regressing" data to represent what happened in the past, you're doing it wrong. I can understand regressing data to guess at what might happen in the future. But what happened, happened. It shouldn't need to be regressed to more closely represent truth, should it? Does Baseball Reference regress Joe Mauer's 2009, because it probably doesn't represent his "true talent level?" No. Nor should it.
While I'm at it...I'll take a shot at the "UZR is accurate in large sample sizes" argument as well. (Unsurprisingly) I don't buy that argument. If it's innacurate in small sample sizes, it is innacurate in larger sample sizes as well. Adding up several small bunches of innacurate data gives me one big bunch of innacurate data, IMO.
Not to mention, if WAR is being used to compare players across eras, there isn't PBP data for any players except those from the last decade or so, so "WAR" for Babe Ruth, for example, is derived without even the flawed data used to derive WAR for today's players.
Personally, I don't understand how smart people pay any attention to it.