Grit Steel Thawed perk does not function for all attacks. There are several things that hit straight through it (This goes for the Follower perk where you split 50% of the damage to your followers, these things hit through that as well). Greater Sabertooth/Panther and Katana are some to name.
Also these two things have insane damage, The Katana can one shot someone with 900 HP if you use the 4 light attacks in the air to finisher combo. The single finish can do upwards of 1100 dmg.
And Greater Sabertooth/Panthers seem to always hit 75% of your HP regardless of your armor/HP/stats… When they do the pounce attack.
Basically everyone on my server is running around with a Katana and a Greater Sabertooth. And if you get hit once by either the Katana finisher or the Sabertooth its basically over. Makes for very uninteresting PvP.