I think that you will be way more successful if you have a solid understanding of usability and a good sense for aesthetics. But I also think that it's more imp

I think that you will be way more successful if you have a solid understanding of usability and a good sense for aesthetics. But I also think that it's more important if you free-lance or work at a small-ish company. In bigger projects and companies, there might be a dedicated person for each aspect of the process, a designer, and front end developer, etc. But the responsibilities are also different in that case. For example, a UX person doesn't just tell you that something isn't as intuitive as it could be or even improve it, but conduct actual research like A/B testing etc. to properly asses the usability, compare different design solutions and so on.