RE: Melee builds
Both Warlock classes have improved reach of their melee, which is an obvious first point in their favor.
Sunsingers get an overshield on hit while Defenders have to kill the enemy in order to get their overshield. Granted Defenders can opt for regenerating their Force Barrier or have lightning fast reloads, still the Sunsinger wins out because it should activate their barrier more often.
Voidwalkers even moreso. They have the excellent Energy Drain tree where on hit reduces the cool-down of grenades. You can use the options additionally reduce Nova Bomb cool-down or receive a significant portion of your health on kill, but that is a bonus -- the effect still activates on hit. Defender Titans don't have that.
If you choose the Nova Bomb cool-down option, and then take Embrace The Void skill perk, Nova Bomb AND grenade hits (not even kills) activate Energy Drain (without even using the melee), which further reduces the cool-down of grenades. You can use The Hunger to further increase the duration of Energy Drain, or wear the Skull of Dire Ahamkara (which I have). This way you can have Energy Drain being activated almost consistently.
So you can theoretically run an all Intellect / Strength build on the Voidwalker and still have ~25 second grenade cool-downs, plus almost stupid-fast Nova Bomb cool-downs and fast melee cool-downs. The Skull of Dire Ahamkara further reduces Nova Bomb cool-downs on grenade kills.
It's a wonderful CQC build and I just started using the Warlock more frequently.