I've done a fair amount of analysis on that reddit post and in general the karmic dice system in the past few years. In summary:

In Early Days (e.g., Patch 4) the Karmic Dice System reduced streaking. A roll of 1-10 would be much more likely to be followed by a roll of 11-20. However, it was inconsistent with a normal distribution. Since then, it was updated to reduce the # of low rolls - using an equation that gives results similar to Rolling With Advantage.

They implemented Karmic Dice because their original default rng system was...terrible. It had the opposite problem: low rolls would be preferentially followed by other low rolls, and vice versa. However, analysis of later patches shows that they stealth fixed this problem, and the Non-Karmic Dice System now produces a roughly uniform distribution of effectively random rolls.

Previous thread on this topic: https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=838587
Looks like my conclusions were:
Originally Posted by mrfuji3
960 Normal Karmic Attack Rolls
Is inconsistent with a normal distribution, showing too many 18s, 19s, and 20s, and too few 2s through 9s.
Average of 12.34 +/- 0.19, compared with an expected average of 10.5

224 Advantage Karmic Attack Rolls
Is overall statistically consistent with a normal distribution. However, there are 35% too few 20s, at 2.3-sigma.
Average of 13.96 +/- 0.28, compared with an expected average of 13.82.

224 Disadvantage Karmic Attack Rolls
Is statistically inconsistent with a normal distribution, with too few 1s through 4s.
Average of 9.45 +/- 0.37, compared with an expected average of 7.17.

With Karmic Dice, higher numbers are progressively more and more likely and on average results in a +2 bonus to your roll.
Having Advantage almost perfectly counteracts the effects of Karmic Dice, except you get less crits.
And more detailed conclusions are here:

Originally Posted by mrfuji3
Combining all the Karmic Die Rolls (735 total rolls)
  • The average is 12.33 +/- 0.22, inconsistent with the expected average of 10.5 at >3-sigma.
  • 18s, 19s, and 20s appear too often (1.5, 1.75, and 2x more frequent than expected for 3, 4.7, and 6.3-sigma differences, respectively.)
  • 2s through 9s don't appear often enough (roughly 60-70% as frequent as expected, for ~2-sigma differences).
  • Somehow, natural 1s occur ~exactly as often as expected, 1/20th of the time. Perhaps Karmic Dice doesn't re-roll 1s...? This would tie in to the larger issue in BG3 where natural 1's override everything.
  • Natural 20s occur about as often as they would in an Advantage system (~10%), but this is not true for other values.
  • The full distribution of rolls is inconsistent with a uniform distribution, with a chi^2 value of 111. WAY larger than the critical value of 30.144 for 19 degrees of freedom
  • Individually, we also reject the hypothesis that each "vs X AC" set of rolls is consistent with uniform, as each has a chi^2 value greater than 30.144.


Combining all the non-Karmic Die Rolls (634 total rolls)
  • The average is 10.42 +/- 0.23, consistent with the expected average of 10.5.
  • All values appear within ~25% (<1.4-sigma) of their expected frequency, with most falling within 15% (<0.8-sigma).
  • The full distribution of rolls is consistent with a uniform distribution, with a chi^2 value of 12.18. Way smaller than the critical value of 30.144 for 19 degrees of freedom
  • Individually, we fail to reject each "vs X AC" distribution as being distinct from a uniform distribution.
  • The higher frequency of natural 20s in the "vs 23 AC" data might just be within expected variation (less than 2-sigma difference; e.g., within the 95% confidence interval).