Different Hex types are not evenly balanced when applied via Hex-on-hit modifiers. Some Hexes like Vulnerability and Despair are much more powerful than others. This is because Hex-on-hit modifiers apply a Hex equivalent to the level 1 version of the Hex skill gem. Some Hexes are very strong at Level 1, relative to their Level 20 power, because they scale in a more gradual way with gem level. This means that in some cases, using Hex-on-hit is too powerful alongside its benefit of also applying Hexes automatically.
We are stabilising the gem scaling rates of Hexes. In general, they should be twice as powerful at gem level 20 than they are at gem level 1.