I don't think its hard, specially if I can understand what's being said, their previous singer (Angela Gossow) had a pretty good growl, and she said that what matters most for her was wether or not the growling is understandable. I don't really care about the technique (because I'm not aware of them, they are useful I know), it just have to be understandable, and have nice tone (not too squeaky or too strong, but that depends on the complexity and weigth level of the instrumental behind it)