I read that Shadow Tanks are quite difficult to heal in Hard Mode.
Is this true ?
Yes, they require a more skilled healer than the other tanks. To put it in simple terms, Vanguards/Guardians will constantly take a medium amount of damage, while Shadows will alternate between taking very little damage and taking *insane* amounts of damage. The total healing required might be similar, but the Vanguard/Guardian is more predictable, and thus easier to heal.
For Vanguards/Guardians, the healer basically goes "healing...healing...healing"
While for Shadows, it's more like "hey, I don't have to do anything... still don't have to do anything... HOLY **** HE JUST LOST 90% HEALTH IN 2 SECONDS"