I had a discussion with a friend about the mechanics of Sekiro, and we came up with some interesting variations in our experience. Some were about the usefulness of certain abilities - his experience was that (for example) using the grappling hook on certain enemies was pointless, as it simply drew him closer, just in time for to be hit by the enemy. I, on the other hand, had a window big enough to attack, then retreat, without being hit. I believe this was simply a timing issue.
The other main issue he had, was to do with the deathblow animation. Personally, I have used this to avoid damage from other enemies. When I'm surrounded by other enemies, I would be grateful for the opportunity to inflict a Deathblow, as it meant I was safe from other incoming attacks. He, however, has not experienced this. It did only happen once, but in a similar situation, just as he entered the deathblow animation, a Club/Mallet Soldier hit him, and he died.
To be honest, I have not played much of the game, and I have only played up to the first boss, so my experience of the mechanics is still fairly fresh. What, including the deathblow animation, causes I-frames, and what is the exact window, and can it be affected by specific enemies?