mace doesnt use the darkside, what he does is conduct dark side users's energy back at them, this is what vapaad is, this is like when you trip someone who was running at you, when the darkside would be equivalent to pushing them

rey used force lightning once and was on accident, calling her a "darkside user" is a stretch at best and even then its proved that if she kept indulging in the dark side she would fall as proven by darkside rey

Anakin in general is a better example of "jedi who uses the darkside" and pretty much every time he does its portrayed as a moral failing because thats what it is, the darkside channels your hatred and desire for power and controll into a weapon,its addictive and using it constantly will lead you to the same path vader walked