I don&#x27;t think Many Worlds&#x2F;Universal Wave Function is actually unpopular amongst physicists. There was a survey (I&#x27;ll have to find it) that showed it was one of the most popular interpretations.<p>It&#x27;s appealing for its mathematical simplicity, giving unitary wave function evolution with the reasonable assumption that the universe has a wave function. It eliminates wave function collapse from the axioms, and possibly eliminates the Born rule.<p>Talking about its &#x27;complexity&#x27; in the &#x27;creation&#x27; of parallel worlds isn&#x27;t much of an issue, you can look at simple systems like the von Neumann measurement model to see how they naturally arise through superposition.<p>The main issue is how the Born rule arises. That is, why the probability of an &#x27;observer&#x27; ending up in a particular branch of the wave function is |psi|^2 isn&#x27;t clear, but then it isn&#x27;t clear what probability means in this context...