All tanks are viable in their current form.
DRK is still a well design class but doesn't perform as well as PLD who excels are everything.
The only problem with DRK is that it's pretty bad until you get your lvl68 and lvl70 skills.
The class is very fun and is pretty engaging.
The Blackest Night (TBN) that you get at lvl 70 is really a game changer for the play style.

Really if you're mainly going to do casual content or even just Extreme Primal, it's not going to affect you.