UX is required for success. You can have the best engineering in the world, but if your customers don't understand your product, cannot work with it, or get frustrated by it, all that beautiful elegant code means nothing. Products need to solve a problem, not add new pain to existing problems.
That does not mean that engineering is less important. Trying to put one above the other is like answering whether the chicken or the egg came first. They are two components that complement each other.