I think most bosses use Physical damage on their hard-hitting single target damage skill, and then have the tendency to throw out AOE Magic dots that DPS generally have to dance around. The issue is, Tanks are used to having to dodge roll or otherwise bypass those red AOEs, but now will have to be much more cognizant of them while still keeping the boss roughly still, because those AOEs will now do a decent chunk more damage with our lowered resistances and less magic mitigation. If it gets to the point where I have to swap to Medium armor for a particularly magic-damage heavy boss, I will let you know, and ZOS as well; hopefully they see fit to fix some of the issues where tanks are expected to dodge roll mechanics, because as it stands now heavy armor feels incredibly restrictive and kind of forces us to block everything.