From the mitigation aspect, part of the reason why monks/paladins/warriors were considered "better" than DK's/druids is their % damage reduction active mitigation mechanics. The simple version is that this reduction is mainly gear independent and very easy to abuse if there is no cap involved to keep it from being abused... also their AM is proactive instead of reactive That being said, that's partly why Guardians/DKs got late buffs since ToT wasn't too kind in that respect, and from the Guardian perspective it made us kind of OP while the blood changes added that extra buff to help with large burst (I tank with both Guardian and Blood DK, which I feel share quite a bit in common despite their differences). Also, with how raids are designed currently, a proactive tanking model is much easier to play than a reactive model, and it takes some healer training to not blow major cooldowns on a Guardian/DK when we're capable of tanking most of our HP in damage at once and instantly recover without external help. In all actuality, this makes Guardians/DK's stronger when things go wrong, since those two tanking classes are very capable of keeping themselves alive with minimal external intervention, while paladins/warriors/monks tend to go "plop."
In a world of tanks caring a lot more about damage than they used to (a little too much imo, but that's the design), damage scaling is a legitimate concern. I've seen similar scaling issues as Vengeance goes up with my Guardian as I do with my DK, as quite a bit of the damage toolkit doesn't scale as well with Vengeance. It also doesn't help that Guardians/DK's are balanced around DoT's, where the other tanks have those big heavy hitting abilities that scale better with Vengeance which can be a detriment to the formerly mentioned classes for threat or burst damage. Nothing's a bigger slap in the face when you spend several GCD's setting up threat/damage when other classes can go lawlKegSmash or something similar and be just as effective in one GCD. Taunt threat change helped with this for single targets, but it's not really going to help in the realm of AoE.
I don't want to get too long with this post, but the short version is that the general perception of blood DKs, as well as Guardians, is far from reality. These tanks are harder to use properly and master with how raids are currently designed compared to warrior/paladin/monk tanks, but they perform just as well (or better in some cases) as other perceived "better" tank classes.