Going into academia if the research is interdisciplinary
We can't really make general statements about one focus of research being better than another. Maybe in this hiring season there are 6 jobs out there perfectly suited to someone doing X, while there are 7 that specifically want (or just prefer) someone doing X+Y. And another year the ranking of numbers might switch. 

Certainly "interdisciplinary research" has a strong positive connotation. My guess is that is because it most commonly entails the combination of a traditional (perhaps stagnant) old field with some sexy new field. The idea is that you are progressing toward "the future" where the field will be evolving in these new directions. While the old pigeonholing of research into particular categories becomes increasingly outdated.

On the other hand if you just combine a couple overcrowded fields that have been around forever you may not gain anything.

One possible drawback I can think of would be in teaching. Even when the hiring committee looks for someone with a lot of breadth, they still may require a lot of depth in their main field. They don't want to be stuck as the only one who can teach that boring old core required course forever.