Originally Posted by
Reinhardt_Azureheim
Unless that max HP boost also fills out the HP gained by this effect, it will generally be awful defensively. Aside from that, this could cause issues for heal checks that cleanse with max HP or do damage based on current health and snapshot in a bad spot of using this action if it runs out in that moment (it will kill the player if that happens, as was evident in Deltascape Savage with Mage's Ballad Troubadour and dropping Defiance in a bad moment back then).
An increase in max HP (with heal based on that increase) and a shield of that exact same increase is fundamentally the exact same. Shields are just temporary health, hence you could call it shield HP. A temporary HP shield by encouraging your allies to brace for a big blow and shake it off is very much in line with WAR identity.
As for the counter attack on Shake it Off, hard pass. If HW parry + Reprisal interaction and StB BRD's self MP refresh usage for Foe's Requieum ever taught me anything is that coupling a direct damage increase for using a "free" mitigation will 9/10 times result in choosing what creates the most damage for oneself rather than what is generally more beneficial to the party. Sure, a potential design avenue and great for healthy optimisation, but parsefrogs will happily act selfish.
Vengeance as the sole tank action with an effect like that (and the only 30% mitigation with a bonus effect in general) is a significant outlier.