My script uses a preview of the actual output from ACR to do the deflicker. From that perspective it may actually be more accurate, not less. The curve for ACR's exposure slider is not perfectly linear and somewhat 'image adaptive' i.e. putting in 1 EV in there doesn't mean ACR is always going to be exactly 1 EV in the output (or not all areas of the image are going to be adjust by exactly 1 EV). So even if you measured 1 EV perfectly in the RAW file, ACR may not actually correct by exactly 1 EV. Additionally with my script you have the flexibility to deflicker when using strong amounts of other ACR settings that tend to cause additional flicker b/c they are 'image adaptive' (such as highlight and shadow recovery and clarity).
If you think of ACR as a black box, the in camera deflicker only has the capability to make a guess as to what it's going to do. My script can actually examine the output and make sure the 'black box' did what it expected, and then make additional corrections. This is the whole reason the script does multiple iterations.
If you use the ufraw workflow, this isn't the case (ufraw is not a 'black box') and the in camera deflicker should be approximately the same.
TBH, I don't think there's going to be really any noticeable difference between the two methods. From my experience, deflickering in post is far more convenient, and you can have a lot smaller interval between pictures (in camera deflicker requires about 5-10s to run), which is important when you get to night time and you need on the order of 30s exposures.
Please feel free to do a comparison though, I could be wrong.