I'll attempt to go into detail after this "over simplified" comparison.
Going from 1-5, where 1 is being bad, and 5 is being good.
Lets say your personal skill at a job is a 1
performance is going to roughly be;
(1)
DRK > PLD > WAR
then going to
(3) PLD > DRK > WAR
then going to
(5)
WAR > PLD > DRK
It's a bit weird, but if you're really bad at the game, you're better off being DRK.
If your mediocre, then PLD will be a better fit.
If you're really good, then WAR will be a better fit.
Of course "difficulty" varies from person to person, but this is a generalization that should fit the largest number of people.
Now for more complexity.
DRK is meant to sacrifice its MP to try and match what the other tanks do.
So it can try to use MP for trash pulls, to gain more mitigation.
It can use MP for mitigating tank busters.
It can use MP to help a party member survive an attack, maybe.
You can use MP to help mitigate magical attacks.
You can spend MP to gain more enmity.
but MP is used for DPS, so you can use MP to raise your DPS.
These are all "Catch up" options, to help you catch up to PLD and WAR.
but in most cases, spending the MP on anything short of DPS, means ur going to lag further behind in DPS from the other tanks. (And atm, DRK is just barely under both tanks in DPS, even if all MP is devoted to DPS)
So if you spend your MP solely for DPS gains, you now lost out on some of your options.
But if you pretend you had those options anyways, DRK has less "filler" mitigation for auto attacks/cleaves, compared to the other tanks.
So when a boss uses a big attack, you're always ready, but for medium/light attacks, you'll find yourself out of mitigations to use. (Luckily a healer can just waste more MP and carry you. Though they would contribute less DPS probably)
You're defensive utility is a single target ability, while PLD and WAR have a similar buff that protects the WHOLE party, not just one person.
So you best be picking the ONE person you feel will die that moment. (Anyone else will just have to die, if they would have died. But luckily not too much content atm requires this save, just unlucky situations. And Healers may or may not have to spend another AoE heal on the group)
You're enmity is lower than the other tanks, unless you spend your MP, then its higher.
But spending your MP is a DPS loss.
Using enmity moves is a DPS loss in all tanks most of the time, but its an even bigger loss on DRK.
If your group doesnt have a NIN, SAM, or WAR for some reason, you'll also be contributing less DPS, since you cant put up a slashing debuff like WAR can.
Normally DRKs living dead is considered pretty useful, but current content makes it worse than hallowed ground/holmgang. (maybe next patch cycle will change that, as its really dependent on when bosses use mechanics)
The hardest content doesnt require the extra defensive utility of PLD/WAR, nor does it require the extra threat/DPS they offer. So its possible to beat content as DRK, but you would be making it slightly harder.
But if you find DRK fun, stick with it. Pick a group of people who dont mind.
But I should mention DRK is better with sustained AoE DPS in dungeons (with WAR being the better burst AoE tank), and people tell me they are great for PvP, though i prefer my PLD for PvP
