For Erlang I would say that the hard part is not the language itself (maybe a bit because it is influenced by prolog) but because of the mental model. Using concurrency and parallelism as core concepts of the language and understanding that you don't need a lot of the external tools you would with the more mainstream languages is what's hard imo.
Until you get complaints that you are not communicating effectively from management and should embrace random fucking calls that could be 2 small paragraphs of text