The Voidwalker isn't crap, it's just outclassed. Sunsingers have much better grenades(yes, this is very important for competitve play) due to Touch of Flame and Viking Funeral and much greater defensive capabilities with Flame Shield and Radiant Skin. Also, nobody uses self res in sweaties, ignoring the fact that you're depriving your team of valuable points they would have gotten from reviving you, radiant skin is much more valued since it makes you even more of a defensive tank when you pop your super and the best warlock players use it to deal with Bladedancers.
Voidwalkers having blink doesn't help since the Bladedancer is the superior blinking subclass due to having higher agility, with again, a superior grenade(arcbolt), superior super which can't be easily blinked through like Nova Bomb can and better class perks like quickdraw, which is an actual game changer in competitive PvP.
Everything that Voidwalker can do other classes can do better, you can get away with it in Trials and normal PvP but you'll just be heavily gimping yourself if you decide to use it against the best players. Of course, there's some very good players who can make it work, but they can make anything work, and even they would perform better if they switched to Sunsinger.