Well poison isn't really hax. It is just most likely the awakened version of the fruit and it is simply too contagious.
Wouldn't say any logia ability is hax for example.
The hax that Haki most likely counters are the more magic ones like sugar ,law teleporting you changing your body and soul ,stuff like that.
Haki protecting you like Luffy's did versus Shinokuni is an entirely different thing and thsi way is that I don't know how much blocks VD.
Logias are indeed not hax but Venom fruit is a paramecia and they are the ones that are the most hax of them all.
Just like virus from DocQ's paramecia that spreads and gets countered by haki, this too will be countered by haki.
The swords of top tier swordsmen have barriers and no touch zones so even if haki doesnt prevent it, barriers do...
I wouldnt be surprised if even when infected by venom it could be stopped by haki...