Doing front-end well involves being able to do a lot reasoning about how human beings will interact with the system, graphic design, and similar sort of skills.

Doing front-end well involves being able to do a lot reasoning about how human beings will interact with the system, graphic design, and similar sort of skills. It's not just the work of making something responsive or organizing SASS/CSS. You're trying to make a coherent experience for a user. I'd expect senior folks to be able to not get confused by an HTML tag, of course, and to be able to build a good end-to-end system, but the really good front-end devs I've worked with think through really subtle details of interaction design -- how long a slider slides, subtle changes to colors during mouse-overs and other actions, and just thousands of details that make a world of difference in how the product feels. Conversely, anyone who's coded for more than a few years should be able to do a database query. But a really senior back-end dev will be able to think through how to, for example, make an architecture capable of streaming through thousands of events per second with 100ms latency, how to shard a system to work in data centers around the world to maintain data coherence while minimizing user latency, or hundreds of similar complex algorithmic tasks. There are plenty of full stack people who can make an app responsive, have a coherent SASS/CSS framework, and efficiently organize a typical database (I can do all of that too, for that matter), but doing either side well takes many years of experience on its own. There are times to go with a generalist. And there are times to go with a specialist.