I like the "Oh Shit" melee concept!
Yeah that's how it works in quite a few other shooter games as well.
But quite often, it is very overpowered (CoD --> 1 stab, 1 kill).
In Planetside 2 for example,
a quick melee attack only takes half of the HP of an enemy player. It works fine as "oh shit", because you can finish off an enemy that you have already reduced to half hp, but after stabbing with your knife, it takes a short time (something like half a second) until you can stab again or are able to continue shooting with your gun.
If you want to 1 hit kill an enemy player with your knife, you need a special one and you have to put your gun away to actually wield it, making yourself vulnerable in case of ranged engagements.
Of course, it is a bit different, because it is PvP and because of the smaller HP pool of targets.
Anyway, i guess the example makes clear what i wanted to say.
Most likely, some people would like to see the option to take melee weapons as primary in ember as well, but there are multiple reasons against that.
One being the limited budged an other one being the (most likely) high HP pool of enemies and (most likely) strong melee attacks from their side.