Application development and UI engineering are two complex, challenging areas within front-end development.
Of course, people will have strengths and weaknesses, and a good team will have practicioners responsible for guiding areas of development. We can’t measure everyone against the same bar, and that’s historically been a mistake of many organizations — exclusively hiring computer science graduates, and wondering why the product is impenetrable to users, or seeking out excellent design talent, and wondering why there’s a lack of automated testing and metrics.
I have the benefit of incredible past mentorship and experiences in both of these areas, and I hold myself and others to a high standard; still, when looking at candidates I wouldn’t stress too much about small gaps in one area or another. But the truth is, over the course of a career, it’s necessary to fill out those gaps and establish a baseline of competency across all areas your core practice interacts with.
Regardless of whether you develop applications or websites, it’s necessary to know the tools available to most effectively solve your problems and create good software. That tool might be a web technology, or it might be streamlining the UX to eliminate a complex and expensive technical problem.
Regardless of specialized roles, experience with all tools available is a must.