I did notice that my stealthed crits seemed to be way off on my last assassin run. Had a few times where even standard melee where surviving my hidden blades when I was 15+ on perilous, or if they did die it'd take until the last hit of an upgraded hidden blades, that would never happen before. Also noticed I sometimes wasn't getting the huge numbers I was used to on commanders with my mighty offense potion + stealth + hidden blades.
similar sort of thing with my archer and mark of death, if I triggered early, I still only seemed be doing double damage at most instead of taking out half their hp as usual. it did occasionally do the damage I expected (ie taking most of rtc health on one good drop with mighty offense) so I figured I was just screwing it up somehow.
did some things get some kind of stealth resistance buff or is it just imagination?