I wouldn't say we're consistently good, per se. At the start of a given expansion, 2-handed Frost is exceptionally good, due to having access to stat inflations that other classes do not, like Killing Machine and Icy Talons. As the expansion moves forward, however, it becomes clear that the amount we benefit from better gear is absolutely horrible compared to other classes, leading to the often-used trigger words "scaling issues."
At the current moment, DK DPS is quite bad, about on par with Retribution Paladins, and lagging behind every other melee DPS spec. Simply put, the same mechanics that put us ahead of the pack near the beginning hurts us near the end. For example, 2-handed frost not only experiences a devaluation of Critical Strike rating of about 20% because of Killing Machine, it gives very little value to Mastery rating, a stat that increases frost damage dealt, due to its signature strike not dealing frost damage, and Haste rating, again, due to Icy Talons, AMS soaking, and a glyph that gives a ridiculous amount of cooldown reduction to AMS.
Frost has suffered from this problem for a very long time, since the start of Cataclysm, which was when KM was changed from affecting Howling Blast and Obliterate to Frost Strike and Obliterate. As it stands on beta, it still to this day suffers from it, with crit and mastery lagging behind by almost 20% compared to versatility, a new secondary stat that just provides a damage increase, a damage taken decrease, and a healing done increase, a stat that no spec should be seeking to stack.
Unholy hasn't had nearly as many problems as Frost has had in the past and present, and has remained a consistently competitive spec (if barely, due to PvP strength regrettably cutting into PvE potential) with little mechanical issues other than clunky AoE rotations. Its disease-centric theme seems to have been lost as of late, but it is our hope that one of our new L100 talents, Necrotic Plague, will provide the gameplay that Unholy deserves and actually saw during 5.2. On beta, however, unholy is looking very weak compared to frost; hopefully a numbers pass will see Noodle stri- I mean, Scourge Strike, as well as Festering Strike buffed to acceptable levels.
Mechanical bullshit aside, Death Knights are pretty straightforward, yes, but my healers in my raid team often cite that the runes/runic power system are too complex. My only suggestion is try it out for yourself once you obtain a level 55 character. I've stuck with it for over 5 and a half years now, and while I've heavily considered rerolling, I will not do so until I extensively dick around with it myself.