Depends on the class, rogues still get hurt a ton from stray arrows, basically glass cannon archetypes are still squishy. Warriors who do not have armor passives will die quite fast as fast as those with
FTFY. Armor is not signifigantly useful in Perilous difficulty. Turn the Bolt / Turn the Blade is, CC is, Cover & Mobility evading attacks is, armor is not.
Turn the Bolt with Turn the blade can reduce a 250+ damage enemy ranged attack, or 450 damage with no ranged defense, to 50-150 damage (depending on ranged defense).
80 constitution promotions would mean you'd have +400hp, on top of the belt which potentially adds another 250hp. 200 hp
So nearly twice the hitpoints of the base class. So yeah, I would imagine you could feel pretty immortal when playing Lego or Templar. I can certainly feel the 18 promotions I have now when I run around as a Templar as I typically am. (But then I run around with +50 guard max and all the passives to make it interesting.)
Unique Health Belt is 200 hp.
The guard max passives isn't all that much, but you may feel it more in threatening and below than perilous.
On a Lego & Templar, you can easily max your front defense to 80%, the default shield providing 30% alone, turn the bolt providing 50% vs ranged, and turn the blade 20% for everything. With those two passives and shields, you already have 50% vs melee and 80% vs ranged, not including everything else involved. A +30-40 promoted constitution (in addition to armor upgrades / passives) means you've maxed melee defense in the front for S&S characters.
I'm currently sitting at +94 Con promote, +11 on armor, and you typically see 12-21 con from passives, in addition to the +12 hp / warrior level. That's > 50% melee resist, and is beyond double the effective melee value of hp (vs no promoted character). On a Katari, I easily break 2k hp. Guard being worth as much as level 1 char's entire hp bar.
Constitution is certainly worth getting. No dps if you're dead. The exact quantifiable benefit of constitution is takes longer to analyze per kit and build, but anything with guard / life drain / healing certainly benefits more.