Interesting, so it seems that OGS (correctly) does not apply any compensation for handicap stones in NZ rules games, yet there is a display bug that erroneously says that compensation has been given.
However, that was not my point. The issue is that (as far as I’m assuming) the rating system treats games under all of the different rules identically, but between the different rules sets, handicap stones may have differing degrees of effectiveness. In isolation, it’s okay for different rules sets to handle handicap differently, but when multiple rules sets are combined under that same rating system, these differences could be a minor problem.
If playing a ranked handicap game, the weaker player should prefer NZ rules and the stronger one should prefer Chinese rules, if they are concerned about rating impact.