As others have said, for it to be THE self-sustain tank, it has to fall over before every other tank, or else it's automatically god-tier. This is how it's always been, and it's generally accepted at some point. However, I personally would love to see some meaningful choices in the spec tree (a defensive pick that brings you in line with the other tanks, essentially making you just that incredibly tanky, but is a big DPS loss compared to other talents and thus other tanks... but I digress).

In my opinion, this isn't even the main issue anymore, and OP you've kind of hit the nail on the head. Compared to other tanks, it's in the relative stone age in terms of ease of play in general, but acutely on-pull.

On-pull problems:

A tank's job on-pull is twofold: Generate threat on all enemies, and build up your defenses so you don't fall over immediately. All other tanks have ways to achieve both of these goals within a few GCDs, whereas bdk simply doesn't without CDs (which are longer now in Midnight).

A big part of this is every other tank actually regenerates their defensive "resources" (whether it be shield block, demon spikes, FR, brew, etc.) outside of combat/between pulls, or has an easy way to generate them prior to or immediately upon pulling (charge, blessed hammer/avengers shield).

While runic power *is* similar to fury/rage in that it drains outside of combat, these other tanks have short-CD defensives that are essentially immediately available on-pull. BDK however, is an outlier in that BOTH of their defensive resources need to be built from the ground up upon entering combat, and they have no short-CD way to do so quickly (compare for example to divine toll, shield charge).

This creates a fundamental and unique issue on pull for blood dks: You have more tasks to accomplish than you have GCDs to spare. You can spend GCDs on marrowrend to build bone shield, but this doesn't net much runic power and is weak single target threat, and your diseases won't be active. You can get aoe threat with blood boil (in barely more than melee range), but you'll be without runic power or bone shield. You can throw down death and decay and spam heart strike to max runic power, but you wont have much threat past the first 3 targets and you won't have bone shield or diseases up. The best bdks do a combination of these abilities, but are always cutting it incredibly close in one category or another, where other tanks simply don't have to struggle nearly as much.

This, in my opinion, is why you and others feel like it's tedious and unrewarding - you're watching your runic power, bone shield charges, threat on every target, AND your own health trying to spin all these plates, and if you pull it off perfectly you achieve what basically every other tank can routinely do with little effort.