I think, that from a immersive point of view, it is ok for me that I can not melee Deathclaws.
There are Stronger faster and there claws are super powerfully.
Bring a gun to a first (Deathclaw) fight! ;)
Maybe with Nerd Rage, Medex, (Power Armor with a mod that grants DR) and other crazy stuff, it still could be possible to melee Deathclaws even with HP that only increase with endurance.
So if you want to battle crazy dangerous animals by hand, make sure you are a berserker, und drug you self into oblivion ;)
(I think that is very ok, because they are high to end game enemys)