Yes, you can go very deep in both front and back end. But the concepts are almost the same... as someone that has had "careers" doing both pixel perfect UIs and large distributed systems on the backend (hundreds of machines, millions of events a second, blahblah).

You can definitely be skilled in a certain tool (language, framework) and that is valuable.

Or you can be the shallow generalist (handyman) running around doing whatever. But the specialist (electrician) is gonna make more than you installing the same outlets.