Paladin misunderstanding.

[Absurdity]

One thing of note in the shelltron vs heart of stone debate is that by popping shelltron a paladin is unable to parry ontop of the mitigation that the cooldown has provided and their benefit of passive blocking is essentially negated for its duration, this also means heart of stone and camo can pair nicely together or just luck.

Another thing to add is that in dungeons and generally all content with big groups of trashmobs block and parry lose a lot of value since critical hits ignore both and we do not have awareness anymore to negate this. So on big pulls a pld can take a lot more damage than the other tanks if your rng is bad.
You could end up with your Shelltron mitigating less damage than HoS and even Camouflage has atleast a 10% damage reduction on top of it's 50% parry.