In the simplest way I could put it, it gets hard enough that most people swap from a Hunter sub to an Etoile sub just so they can take another hit instead of being killed right away when Iron Will doesn't proc in time or when Automate Halfline fails to trigger because you got one-shotted.
This isn't true. People take Etoile sub because it's more damage. Hunter sub is still very tanky -- Flash Guard points apply at all times, even when you don't have Guard Stance activated.
This is why BoEt and BrEt don't get used (except BrEt for bow only). Their damage is not good, and you're better off using BoHu or BrHu. Same thing for Fighters utilizing Double Sabers -- FiHu does more damage than FiEt for that, and FiEt is only better for Knuckles/Twin Daggers.
FoEt is even in a similar situation. FoTe only beats FoEt when fighting light/dark/wind elemental weakness enemies with the respective element techs, and its only by a small margin of like 3%. FoEt compounds are stronger unless the enemy is weak to both elements of the compound, which is super rare - this only happens on like, Gemini. And Fire, Ice, and Lightning Techs (although Fire doesn't get used in UH other than for Status inflicting, i can explain why another time) all do a gigantic amount more dps at all times. This wouldn't work if there weren't new ways to manage PP in EP6, but thanks to Genon + strong weapons with potentials that reduce PP consumption and such and even unit SSAs that help with PP management, FoEt becomes way more viable. Hell this is even backed up by the drop in light weak enemies, wind techs being mostly bad, and dark techs being kinda hard to use especially on mobile enemies.
The extra tankiness is just the cherry on top. And hell, even if HuFi did a little more damage than HuEt in terms of multipliers, HuEt would probably be better because of the more lenient combo bonus allowing you to repeat a PA 3 times in a row while maintaining that damage bonus, whereas Fi sub only allows once before losing that extra damage.
TL;DR: Etoile is picked because of damage, not tank. The tank is a bonus.