Every single job in the game is afected with the rule of always be casting, hit all your GCD under your buff windows and keep everything on recast, DRK is not special just bcs he have 2 resources in 1 combo, if you mess up you need to recalculate for the next buff window not the entire fight at all, thats with all respect a over exageration considering how little resources generate right now and how wide the downtime is, you have plenty of time to know if you should use a edge early or not depending of your resources aviable.
If you missed a few hits before the boss jump thats nothing compared to start no mercy window with GNB and miss everything with no return or inner release Window that cost WAR more dps and infuriate CD reduction, DRK mistakes are way less severe than the other tanks due how the job have no inner mechanics whatssoever to keep in check, resource generation and spending and keep GCD always runing is a norm for everyone as you say and at contrary than others jobs like the rest of the tank who have to delay the whole burst window or lose it complety DRK biggest mistake is only not having living shadow have his full duration uptime since everything else can be recolocated easily after and prepare your resources for the next buff window resulting in mimnimun loss compared to the other tanks mechanics, there is a reason why is so easy archive high score with DRK on certain web.
DRK basic (and mostly advanced) optimization is reduced on hit 5 GCD under BW and Delirium and keep everything on recast wich thats a task that only happens during 10s every min, not in vain DRK is considering with WAR the most brain dead jobs of the game right now by the raiding comunity due the little requeriments of optimization and the absurd amounts of downtime to do whatever you need, in fact DRK is picked bcs TBN is just to strong and you have so little to do with the job you can complete savage on easy mode focusing way more on combat mechanics at the same time many says the job is boring to play when you master it.