There are some times when the assassin is not good enough for the basic content (crusher of foremans cannot be reliably solo tanked by an assassin tank for instance because they suck too much). The jugg is kind of sucky and complicated but it can still handle the same stuff as a PT.
Methinks someone has an overinflated sense of the potency of PT/VG tanks and the "suckiness" of every other tank. I have never had a problem with tanking any content on my Shadow tank, including Foreman Crusher, and I've tanked Ops with all 3 of the classes. I stand by my descriptions.
The difference at the top tier is largely based upon how good you are at using and abusing the variable mechanics of your class. In general, this means CD management, which is why PT/VGs are said to have the lowest "tank ceiling": there just isn't that much to them beyond their passive and standard mitigation mechanics. Once you figure out how to faceroll your attacks while watching your ammo bar, you're pretty much as good as a PT/VG tank can get. For Guardians, once you've figured out how to manage your Focus/Rage, you're set, but you can still learn to eke a bit more out of your CDs and other mechanics when possible (playing around with Guardian Leap, learning to use FD as a tank CD, learning to cycle your 3 min CDs so that you actually use them and/or they're up when you need them since most high damage mechanics operate on a ~2 min cycle). It really isn't all that hard to know how to play a Shad/Sin (keep KW up while F/T attacks are heading at you, Project and Slow Time on CD, 3xHS TkT immediately when you get the stacks, only use DS to stop yourself from wasting Force regen), but, beyond that, there are a lot more mechanics within the realm of playing a Shadow tank that can be tweaked and abused to improve performance further: Shadow CDs require you actually know what kind of attacks are being used to best benefit from them (Resilience on Toth/Firebrand is kinda pointless and Deflection on Zorn/Stormcaller is idiotic) and their low CDs mean that, unless you're using them a lot, you're not getting everything out of them you could; KW isn't an attack like Riposte so you don't simply spam it on CD (you could, but it's gonna eat up Force; it's better to simply pay attention to it; it's not even needed on some fights/phases so you shouldn't use it); There are times when you really don't want to use TkT even though you've got 3 stacks because any movement or knockback is gonna render all of that time and Force invested in it wasted (TkT interrupted by movement phases, knockback and/or doom patches require you know fights and abuse some of your abilities to get around them).
I've played all the tank classes, and played *alongside* all of the tank classes. PT/VGs are either good or bad. There isn't enough there to really make a big difference once you're not acting like an idiot. The other tanks, however, have enough mechanics that there actually *is* a difference between someone that understands the basics of the class and someone that knows exactly how to play the class to maximize effectiveness. It's part and parcel with the amount of skill required to play the classes themselves, and I bring this up because the hecticness of a class is pretty tightly bound with the amount of skill required to play it well.
If all you're looking for is the simplest tank in the entire game, go with a PT/VG. As long as you're not an idiot, you'll tank just fine. If you're willing to put in some effort to learn the class, you won't even notice any real "hecticness" on a Jugg/Guardian; once you learn to manage your resources, it's pretty much second nature when playing it. No matter how you play it (as long as you're playing it properly instead of simply expecting everything to play like a VG), a Shad/Sin is going to exist on the hectic side of the sliding-scale between faceroll and hectic.
In all of this discussion, I think it's also important to point out that there is a difference in the "type" of hectic-ness that Guard/Juggs and Shad/Sins exhibit. Guardians are hectic because decisions made now have repercussions later on (Resource management) and more buttons to press as part of their baseline performance (purely for basic play, you've got roughly 11 abilities to interact with regularly compared to the 6-7 that the other tanks do). Shadows are hectic because you have to pay attention to a lot more (buffs at certain levels, types of attacks going against you, variable CD abilities, etc). It's a different type of hectic-ness compared to playing a Guard/Jugg (which is a bit of a culture shock when you're going from one to the other). If you're used to managing more buttons more often and are simply thrown off by how much attention to stuff aside from yourself you have to pay while playing a Shadow, Guardian/Jugg might be appropriate for you're used to that type of hectic-ness. Of course, if any type of hectic-ness is contradictory to your enjoyment of the game, PT/VG has none of it whatsoever.