If done wrong, it's bad because you're on a desktop and seeing a mobile interface when you shouldn't. The Charles Schwab website is an example of that. If done right, it should feel natural on any size screen, and you should have visual cues as you resize. Again, in many cases it's not about hiding UI elements completely, but about simplifying them so they degrade from word+icon, to smaller font, to just icon, to drop menu. Although it's hardly an example of cutting edge design, the overall AWS console handles this pretty well, shuffling what you need into smaller compartments. (Some individual services do better than others). If you open 4 of them on a desktop it's still fairly easy to find what you need, and hasn't yet degraded all the way to a mobile interface. In my app designs there are at least three levels, usually four.