Consider two builds, one with 1000 hit points and no damage mitigation and the other with 500 hit points and 50% damage mitigation. Using the formula EHP=HP/M we get identical EHP of 1000.
The argument is that these two builds are then equivalent.
Now place them into an encounter where the monster does 750 damage. Make maximum healing 250 for each character and assume that the rate of damage is the same for both so that two heals take place before each hit.
Here is what happens:
A with 1000 hp
1000-750=250+500=750-750=0, character is unconcious.
B with 500 hp
500-375=125+500=500-375=125+500=500, etc. (Note: HP cannot exceed 500).
These two builds with exactly the same EHP are not equivalent builds because one dies on the second hit while the other continues fighting indefinitely.
We only see this if we take EHP and derive HTK from it. Stopping short does not reveal the whole story.
And, HTK alone does not reveal the whole story either. It only allows us to get to the external information concerning healing.
This is why EHP cannot be used to compare builds for equivalency. It is two steps removed from the actual information needed. HTK may be imperfect but it is closer to the actual information we must have to make an equivalency determination.
