Macro Gameplay
I'd like to think I'm pretty good at PvP, but in general it can feel pretty hard to parse exactly what's going on. Dark Knight allows for an easily understood game plan that can be countered or followed by anyone paying attention.
Following the plan is simple: hold your burst, wait for the Dark Knight to go in, follow up with AOE damage. While some jobs contribute more direct damage than others, every job can participate in this plan. And not all jobs need to contribute right away to the big slam. In the aftermath, 10-20 seconds later, jobs that deal hefty single target damage or have big DoT damage are given a chance to shine by cleaning up a weakened team.
Countering a Dark Knight burst is difficult. I say this as a good thing; Frontlines and PvP in general encourages you to be active and get the drop on your enemies. But while specific LBs can outright counter a Dark Knight succ (Dragoon/Sage/Samurai/Paladin/Scholar/etc.), there's quite a few other things that let you survive and turn momentum on the enemy team:
1) Spacing, spacing, spacing. When in an engagement there are two positions you need to consider: stay as far away from your teammates as possible (to receive/give heals) and as close to your goal as possible.
2) Disrupting burst. This is the realm of specific jobs with knockbacks, pulls, AoE stuns, and is a more reactive approach to defense. As soon as you see that Dark Knight moving towards you, stun them and move them away from your team. Monk/Reaper/Dancer LB them or their team, stun their AoE bursters, distract them with Viper's tankiness beforehand, anything to keep them from going 3...2...1...now!
3) Take initiative. All of these require you to keep an active eye on your enemy and your team, during which time you may see an opening. Set up a hotbar to call out when and where to hit, and do it before the enemy hits you.
All in all, Dark Knight encourages people to pay attention to the larger game at hand: when and where is my team going to attack, and when is the enemy team going to attack. People who lack this game sense will get blindsided by the Dark Knight burst, but also any other burst in the game. Which brings me to my last point!
Learning Curve
Dark Knight is the first job you should learn to look out for, and the easiest to read. Read their posture -- do they have sprint on? Do they have one of my teammates targeted?
You start doing this on a single class, the most important one in the game. Then you learn to read the other players on their team: the Dragoons, the Astros, the Summoners. Needing to understand and adjust to what your enemy is doing isn't unfair, it's inherent to video games.
It gets you in the mindset of reading the macro-gameplay of Frontlines -- when to pull back, when to attack, how to communicate with your team. It's healthy for the game, as the threat of a high-paced attack is exciting to wield and dodge.