"a: A heavy armored Space Marine / Chaos Space Marine
b: Made completely out of metal Necron?
c: ...Orks? Do they look tasty at all?
d: Eldar's armor is made up of their "crystal" thingy. Imagine chewing glass D:
e: Nearly un-armored Imperial Guard?"
a: Stronger biomass to be consumed.
b: Tyranids do not fight Necrons.
c: The Hive Mind does not care about taste, it is filled with eternal hunger.
d: Even Eldar have biomass.
Also Tyranids do not just eat the meat of their enemies, they devour the whole PLANET! They devour the atmosphere and every single drop of biomass, leaving the world a dead rock.
Also the Hive Mind is not a "psycho entity far far away", it's literally the brain of the Tyranids, it IS the Tyranids! Every Tyranid is a part of the Hive Mind's body. It is not a entity controlling a race, it is a big creature(The Hive Mind) moving it's body (the Tyranids)over the galaxy, devouring it.