Reaper Burst is super slow because of the silly shit that is collecting eight stacks. Mnk suffers from this to a lesser extent due to the big button being locked behind six other presses. That said this isn't even the biggest problem with the job.
Imagine any other class having their Limit break struck down with a single OGCD. Not to mention the fact that the timer is so low on your stacks in enshroud you can barely get half of it used between CC and people scattering like roaches.
Communio is basically designed to get you killed.
Anyone who says Dark Knight(lol death knight huh?) is less survivable just doesn't know how to play it. You can't walk into the enemy team solo and survive indefinitely on anything but a paladin with a limit break and that is by design. Imagine a world where tanks got to do basically all the roles... never mind that's how wow pvp tanks work.