Very true, like with most things in life, balance is key. Full stack developer who's emphases is X is a grate candidate, assuming X is what you are looking for. Full stack developer who is "great" at everything is probably not so much. My personal attitude in life is to know as much as I care about subjects which are my core competency and are most interesting to me, and know enough about other fields to be able to competently hire an expert in those fields, and be able to spot instances when that expert is bullshitting me.
