The main downside I've experienced is that managers (and sometimes co-workers) react negatively to anyone who has skill in more than one area. They become obses

The main downside I've experienced is that managers (and sometimes co-workers) react negatively to anyone who has skill in more than one area. They become obsessed with making it seem as though you're an idiot and that you aren't actually skilled in multiple areas. Also, managers at larger companies often pretend they're trying to hire Full-Stack Engineers, but are really trying to hire someone to write Javascript (that is, they are full of it and start trying to box you in right away). It's true that more time has to be spent keeping up with advancements, but it doesn't take that much longer than paying attention to one area, as things don't change that often and you don't have to "level up" immediately.