Blizzard has said time in and time out that they would like Warlocks to be the "tanky" caster while casters like mage be the "mobile caster." This inherently puts Warlocks at a disadvantage when dealing with mechanics in a raiding (SPECIFICALLY RAIDING, I DO NOT PVP ENOUGH TO MAKE STATEMENTS BASED ON IT) environment.
To speak very generally, raiding mechanics can more or less be split into two categories: avoidable and unavoidable. Things like raid wide pulses from the boss are unavoidable, circles the boss puts under the raid are avoidable. You get the idea. Typically those unavoidable mechanics are not the mechanics to wipe a raid, as they are unavoidable. A healer's job is to heal the unavoidable damage such as mechanics the boss does that hits everyone and the bosses melee swings, and a dps and tanks job is to minimize that damage while doing as much damage or taking the least damage as possible, respectively.
This is where the core flaw in the "tanky caster" comes into play. Because Blizzard has provided less tools for the tanky caster to avoid those mechanics, they are either forced to take that mechanics or be penalized for avoiding it. The mobile caster has to avoid that avoidable mechanic, as they will take comparably more damage than the tanky caster, so they are provided tools that either entirely negate that mechanic or avoid it. Since Blizzard likes to compare us to mages, I'll use them as an example: Blink and Ice Block are very powerful tools. Being able to instantly avoid a mechanic or negate the damage entirely. While Warlocks have tools such as Soul Leech and Dark Pact, all these serve to do is mitigate that damage.
Mitigate is the key word. Warlock's "tankiness" only serves to mitigate that damage. We do not have the tools provided baseline to avoid these mechanics, only to assist in mitigating the damage. In a progression environment where taking the least amount of damage as possible is desirable, what does that tanky caster bring over the mobile caster? One has tools to avoid mechanics, while the other has tools in reducing the amount of damage they take (or they can choose to avoid the mechanics, but are punished much more so because of the limited mobility). So while Warlocks will take less unavoidable damage than mages, they are either forced to take more avoidable damage or be penalized for avoiding those mechanics.
On a side note, all the mechanics in a raid environment generally don't allow "tanking through it" as they either silence/one shot kill you/permanent ground effects. More than likely you're going to have to move out of it anyways. But thats a different QQ for a different day.
This "tanky caster" focus is flawed. It's okay to have key distinctions of classes to differentiate them, but making an entire class built around one thing is bad design.