Seki is right when he says Melee and Sneak Melee cannot be balanced the same way, as they do not have same purpose/gameplay.
But one of the issue is the same, no aiming time for aimed attacks with melee weapons (sneak or not), that makes Melee & Sneak Melee 90% of insta win as soon as they reach their opponent.
However how do you balance it ? If melees didn't KO, they would be weak, if they could run faster, they would be OP. So atm, melees are very strong for 1v1 because it's easy to cut los, but if one doesn't want to fight, he can avoid the other. In ZC, they are a bit too situational, they are very strong in buildings, and very weak in opened areas. I think it would be very hard to balance them without making them useless/OP.
About the Melee Sneak, well they are quite OP on 1v1 as you can choose your fights (thanks to sneak) and you can close the distance for the same reasons. Also your weapon skill is based on a secondary skill, so you can lower tagged weapon skill to have 200 in all secondary skills without any sacrifice. The dam is insane and you can hit twice with 100 AP (aimed attacks of course). The thing is, sneak should be strong in 1v1, but atm they kill every solo a bit too easily. Also they kill fast, but they die fast too as they are wearing leather jackets.